diff --git a/examples/qpdfjob-remove-annotations.cc b/examples/qpdfjob-remove-annotations.cc
index fd8754d5..5334e7dc 100644
--- a/examples/qpdfjob-remove-annotations.cc
+++ b/examples/qpdfjob-remove-annotations.cc
@@ -11,7 +11,7 @@
 // The example is a full copy of the qpdf program modified to allways remove all
 // annotations from the final output.
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 static void
 usageExit(std::string const& msg)
diff --git a/include/qpdf/Pl_Buffer.hh b/include/qpdf/Pl_Buffer.hh
index e2f71094..fcef0c56 100644
--- a/include/qpdf/Pl_Buffer.hh
+++ b/include/qpdf/Pl_Buffer.hh
@@ -43,7 +43,7 @@ class QPDF_DLL_CLASS Pl_Buffer: public Pipeline
 {
   public:
     QPDF_DLL
-    Pl_Buffer(char const* identifier, Pipeline* next = 0);
+    Pl_Buffer(char const* identifier, Pipeline* next = nullptr);
     QPDF_DLL
     virtual ~Pl_Buffer();
     QPDF_DLL
diff --git a/include/qpdf/Pl_DCT.hh b/include/qpdf/Pl_DCT.hh
index 543967ec..9352091f 100644
--- a/include/qpdf/Pl_DCT.hh
+++ b/include/qpdf/Pl_DCT.hh
@@ -57,7 +57,7 @@ class QPDF_DLL_CLASS Pl_DCT: public Pipeline
         JDIMENSION image_height,
         int components,
         J_COLOR_SPACE color_space,
-        CompressConfig* config_callback = 0);
+        CompressConfig* config_callback = nullptr);
 
     QPDF_DLL
     virtual ~Pl_DCT();
@@ -91,7 +91,7 @@ class QPDF_DLL_CLASS Pl_DCT: public Pipeline
             JDIMENSION image_height = 0,
             int components = 1,
             J_COLOR_SPACE color_space = JCS_GRAYSCALE,
-            CompressConfig* config_callback = 0);
+            CompressConfig* config_callback = nullptr);
         Members(Members const&) = delete;
 
         action_e action;
diff --git a/include/qpdf/Pl_QPDFTokenizer.hh b/include/qpdf/Pl_QPDFTokenizer.hh
index ce3615ec..0b8cafc2 100644
--- a/include/qpdf/Pl_QPDFTokenizer.hh
+++ b/include/qpdf/Pl_QPDFTokenizer.hh
@@ -51,7 +51,7 @@ class QPDF_DLL_CLASS Pl_QPDFTokenizer: public Pipeline
     Pl_QPDFTokenizer(
         char const* identifier,
         QPDFObjectHandle::TokenFilter* filter,
-        Pipeline* next = 0);
+        Pipeline* next = nullptr);
     QPDF_DLL
     virtual ~Pl_QPDFTokenizer();
     QPDF_DLL
diff --git a/include/qpdf/QPDF.hh b/include/qpdf/QPDF.hh
index 5567a686..03490af7 100644
--- a/include/qpdf/QPDF.hh
+++ b/include/qpdf/QPDF.hh
@@ -83,7 +83,7 @@ class QPDF
     // interpreted as a raw encryption key. See comments on
     // setPasswordIsHexKey for more information.
     QPDF_DLL
-    void processFile(char const* filename, char const* password = 0);
+    void processFile(char const* filename, char const* password = nullptr);
 
     // Parse a PDF from a stdio FILE*.  The FILE must be open in
     // binary mode and must be seekable.  It may be open read only.
@@ -96,7 +96,7 @@ class QPDF
         char const* description,
         FILE* file,
         bool close_file,
-        char const* password = 0);
+        char const* password = nullptr);
 
     // Parse a PDF file loaded into a memory buffer.  This works
     // exactly like processFile except that the PDF file is in memory
@@ -107,14 +107,14 @@ class QPDF
         char const* description,
         char const* buf,
         size_t length,
-        char const* password = 0);
+        char const* password = nullptr);
 
     // Parse a PDF file loaded from a custom InputSource.  If you have
     // your own method of retrieving a PDF file, you can subclass
     // InputSource and use this method.
     QPDF_DLL
     void
-    processInputSource(std::shared_ptr<InputSource>, char const* password = 0);
+    processInputSource(std::shared_ptr<InputSource>, char const* password = nullptr);
 
     // Create a PDF from an input source that contains JSON as written
     // by writeJSON (or qpdf --json-output, version 2 or higher). The
diff --git a/include/qpdf/QPDFCryptoImpl.hh b/include/qpdf/QPDFCryptoImpl.hh
index 5345b45b..3fd64d3a 100644
--- a/include/qpdf/QPDFCryptoImpl.hh
+++ b/include/qpdf/QPDFCryptoImpl.hh
@@ -80,7 +80,7 @@ class QPDF_DLL_CLASS QPDFCryptoImpl
     virtual void RC4_process(
         unsigned char const* in_data,
         size_t len,
-        unsigned char* out_data = 0) = 0;
+        unsigned char* out_data = nullptr) = 0;
     QPDF_DLL
     virtual void RC4_finalize() = 0;
 
diff --git a/include/qpdf/QPDFJob.hh b/include/qpdf/QPDFJob.hh
index 2a82d61c..7396cd6a 100644
--- a/include/qpdf/QPDFJob.hh
+++ b/include/qpdf/QPDFJob.hh
@@ -497,7 +497,7 @@ class QPDFJob
 
     // Helper functions
     static void usage(std::string const& msg);
-    static JSON json_schema(int json_version, std::set<std::string>* keys = 0);
+    static JSON json_schema(int json_version, std::set<std::string>* keys = nullptr);
     static void parse_object_id(
         std::string const& objspec, bool& trailer, int& obj, int& gen);
     void parseRotationParameter(std::string const&);
diff --git a/include/qpdf/QPDFObjectHandle.hh b/include/qpdf/QPDFObjectHandle.hh
index ee424e39..198ca42e 100644
--- a/include/qpdf/QPDFObjectHandle.hh
+++ b/include/qpdf/QPDFObjectHandle.hh
@@ -526,7 +526,7 @@ class QPDFObjectHandle
     QPDF_DLL
     void parsePageContents(ParserCallbacks* callbacks);
     QPDF_DLL
-    void filterPageContents(TokenFilter* filter, Pipeline* next = 0);
+    void filterPageContents(TokenFilter* filter, Pipeline* next = nullptr);
     // See comments for QPDFPageObjectHelper::pipeContents.
     QPDF_DLL
     void pipePageContents(Pipeline* p);
@@ -538,7 +538,7 @@ class QPDFObjectHandle
     // contents. This can be used to apply a TokenFilter to a form
     // XObject, whose data is in the same format as a content stream.
     QPDF_DLL
-    void filterAsContents(TokenFilter* filter, Pipeline* next = 0);
+    void filterAsContents(TokenFilter* filter, Pipeline* next = nullptr);
     // Called on a stream to parse the stream as page contents. This
     // can be used to parse a form XObject.
     QPDF_DLL
diff --git a/include/qpdf/QPDFOutlineObjectHelper.hh b/include/qpdf/QPDFOutlineObjectHelper.hh
index c8b82a80..507a4a51 100644
--- a/include/qpdf/QPDFOutlineObjectHelper.hh
+++ b/include/qpdf/QPDFOutlineObjectHelper.hh
@@ -42,7 +42,7 @@ class QPDFOutlineObjectHelper: public QPDFObjectHelper
     {
         // This must be cleared explicitly to avoid circular references
         // that prevent cleanup of pointer holders.
-        this->m->parent = 0;
+        this->m->parent = nullptr;
     }
 
     // All constructors are private. You can only create one of these
diff --git a/include/qpdf/QPDFPageObjectHelper.hh b/include/qpdf/QPDFPageObjectHelper.hh
index ebb9e87e..9f647c70 100644
--- a/include/qpdf/QPDFPageObjectHelper.hh
+++ b/include/qpdf/QPDFPageObjectHelper.hh
@@ -320,12 +320,12 @@ class QPDFPageObjectHelper: public QPDFObjectHelper
     // examples/pdf-count-strings.cc for an example.
     QPDF_DLL
     void
-    filterContents(QPDFObjectHandle::TokenFilter* filter, Pipeline* next = 0);
+    filterContents(QPDFObjectHandle::TokenFilter* filter, Pipeline* next = nullptr);
 
     // Old name -- calls filterContents()
     QPDF_DLL
     void filterPageContents(
-        QPDFObjectHandle::TokenFilter* filter, Pipeline* next = 0);
+        QPDFObjectHandle::TokenFilter* filter, Pipeline* next = nullptr);
 
     // Pipe a page's contents through the given pipeline. This method
     // works whether the contents are a single stream or an array of
diff --git a/include/qpdf/QPDFWriter.hh b/include/qpdf/QPDFWriter.hh
index 15b5579f..8305974c 100644
--- a/include/qpdf/QPDFWriter.hh
+++ b/include/qpdf/QPDFWriter.hh
@@ -539,7 +539,7 @@ class QPDFWriter
         friend class QPDFWriter;
 
       public:
-        PipelinePopper(QPDFWriter* qw, std::shared_ptr<Buffer>* bp = 0) :
+        PipelinePopper(QPDFWriter* qw, std::shared_ptr<Buffer>* bp = nullptr) :
             qw(qw),
             bp(bp)
         {
diff --git a/include/qpdf/QUtil.hh b/include/qpdf/QUtil.hh
index 5291fe09..2f7f15b2 100644
--- a/include/qpdf/QUtil.hh
+++ b/include/qpdf/QUtil.hh
@@ -245,7 +245,7 @@ namespace QUtil
     // Returns true iff the variable is defined.  If `value' is
     // non-null, initializes it with the value of the variable.
     QPDF_DLL
-    bool get_env(std::string const& var, std::string* value = 0);
+    bool get_env(std::string const& var, std::string* value = nullptr);
 
     QPDF_DLL
     time_t get_current_time();
diff --git a/libqpdf/QUtil.cc b/libqpdf/QUtil.cc
index 4f9c1021..a542736d 100644
--- a/libqpdf/QUtil.cc
+++ b/libqpdf/QUtil.cc
@@ -836,8 +836,8 @@ char*
 QUtil::getWhoami(char* argv0)
 {
     char* whoami = nullptr;
-    if (((whoami = strrchr(argv0, '/')) == NULL) &&
-        ((whoami = strrchr(argv0, '\\')) == NULL)) {
+    if (((whoami = strrchr(argv0, '/')) == nullptr) &&
+        ((whoami = strrchr(argv0, '\\')) == nullptr)) {
         whoami = argv0;
     } else {
         ++whoami;
diff --git a/libqpdf/qpdf/Pl_SHA2.hh b/libqpdf/qpdf/Pl_SHA2.hh
index d5b2e547..99a19a37 100644
--- a/libqpdf/qpdf/Pl_SHA2.hh
+++ b/libqpdf/qpdf/Pl_SHA2.hh
@@ -20,7 +20,7 @@
 class Pl_SHA2: public Pipeline
 {
   public:
-    Pl_SHA2(int bits = 0, Pipeline* next = 0);
+    Pl_SHA2(int bits = 0, Pipeline* next = nullptr);
     virtual ~Pl_SHA2() = default;
     virtual void write(unsigned char const*, size_t);
     virtual void finish();
diff --git a/libqpdf/qpdf/QPDFArgParser.hh b/libqpdf/qpdf/QPDFArgParser.hh
index 89c7acc0..759ad148 100644
--- a/libqpdf/qpdf/QPDFArgParser.hh
+++ b/libqpdf/qpdf/QPDFArgParser.hh
@@ -177,9 +177,9 @@ class QPDFArgParser
     {
         OptionEntry() :
             parameter_needed(false),
-            bare_arg_handler(0),
-            param_arg_handler(0),
-            invalid_choice_handler(0)
+            bare_arg_handler(nullptr),
+            param_arg_handler(nullptr),
+            invalid_choice_handler(nullptr)
         {
         }
         bool parameter_needed;
diff --git a/libqpdf/qpdf/QPDFCrypto_openssl.hh b/libqpdf/qpdf/QPDFCrypto_openssl.hh
index eae69ab1..0d07b99b 100644
--- a/libqpdf/qpdf/QPDFCrypto_openssl.hh
+++ b/libqpdf/qpdf/QPDFCrypto_openssl.hh
@@ -39,7 +39,7 @@ class QPDFCrypto_openssl: public QPDFCryptoImpl
     void RC4_process(
         unsigned char const* in_data,
         size_t len,
-        unsigned char* out_data = 0) override;
+        unsigned char* out_data = nullptr) override;
     void RC4_finalize() override;
 
     void SHA2_init(int bits) override;
diff --git a/libtests/aes.cc b/libtests/aes.cc
index 4541fffe..5ae8152a 100644
--- a/libtests/aes.cc
+++ b/libtests/aes.cc
@@ -29,9 +29,9 @@ main(int argc, char* argv[])
 {
     bool encrypt = true;
     bool cbc_mode = true;
-    char* hexkey = 0;
-    char* infilename = 0;
-    char* outfilename = 0;
+    char* hexkey = nullptr;
+    char* infilename = nullptr;
+    char* outfilename = nullptr;
     bool zero_iv = false;
     bool static_iv = false;
     bool disable_padding = false;
@@ -65,7 +65,7 @@ main(int argc, char* argv[])
             usage();
         }
     }
-    if (outfilename == 0) {
+    if (outfilename == nullptr) {
         usage();
     }
 
@@ -81,14 +81,14 @@ main(int argc, char* argv[])
         t[1] = hexkey[i + 1];
         t[2] = '\0';
 
-        long val = strtol(t, 0, 16);
+        long val = strtol(t, nullptr, 16);
         key[i / 2] = static_cast<unsigned char>(val);
     }
 
     Pl_StdioFile* out = new Pl_StdioFile("stdout", outfile);
     Pl_AES_PDF* aes = new Pl_AES_PDF("aes_128_cbc", out, encrypt, key, keylen);
     delete[] key;
-    key = 0;
+    key = nullptr;
     if (!cbc_mode) {
         aes->disableCBC();
     }
diff --git a/libtests/arg_parser.cc b/libtests/arg_parser.cc
index 6b4c829f..c9474d55 100644
--- a/libtests/arg_parser.cc
+++ b/libtests/arg_parser.cc
@@ -51,7 +51,7 @@ ArgParser::initOptions()
     ap.addBare("potato", b(&ArgParser::handlePotato));
     ap.addRequiredParameter("salad", p(&ArgParser::handleSalad), "tossed");
     ap.addOptionalParameter("moo", p(&ArgParser::handleMoo));
-    char const* choices[] = {"pig", "boar", "sow", 0};
+    char const* choices[] = {"pig", "boar", "sow", nullptr};
     ap.addChoices("oink", p(&ArgParser::handleOink), true, choices);
     ap.selectHelpOptionTable();
     ap.addBare("version", [this]() { output("3.14159"); });
diff --git a/libtests/numrange.cc b/libtests/numrange.cc
index c0bf5bd7..ded03324 100644
--- a/libtests/numrange.cc
+++ b/libtests/numrange.cc
@@ -4,7 +4,7 @@
 static void
 test_numrange(char const* range)
 {
-    if (range == 0) {
+    if (range == nullptr) {
         std::cout << "null" << std::endl;
     } else {
         std::vector<int> result = QUtil::parse_numrange(range, 15);
diff --git a/libtests/predictors.cc b/libtests/predictors.cc
index fd1cc2ad..d7825bc6 100644
--- a/libtests/predictors.cc
+++ b/libtests/predictors.cc
@@ -21,7 +21,7 @@ run(char const* filename,
     FILE* in = QUtil::safe_fopen(filename, "rb");
     FILE* o1 = QUtil::safe_fopen("out", "wb");
     Pipeline* out = new Pl_StdioFile("out", o1);
-    Pipeline* pl = 0;
+    Pipeline* pl = nullptr;
     if (strcmp(filter, "png") == 0) {
         pl = new Pl_PNGFilter(
             "png",
diff --git a/libtests/qutil.cc b/libtests/qutil.cc
index a6f1282a..be5b6b1b 100644
--- a/libtests/qutil.cc
+++ b/libtests/qutil.cc
@@ -489,7 +489,7 @@ same_file_test()
     assert_same_file("qutil.out", "qutil.out", true);
     assert_same_file("qutil.out", "other-file", false);
     assert_same_file("qutil.out", "", false);
-    assert_same_file("qutil.out", 0, false);
+    assert_same_file("qutil.out", nullptr, false);
     assert_same_file("", "qutil.out", false);
 }
 
diff --git a/libtests/random.cc b/libtests/random.cc
index 40ef7d47..899c264c 100644
--- a/libtests/random.cc
+++ b/libtests/random.cc
@@ -56,7 +56,7 @@ main()
     if (!((buf[0] == 0) && (buf[1] == 1) && (buf[2] == 2) && (buf[3] == 3))) {
         std::cout << "fail: bogus random didn't provide correct bytes\n";
     }
-    QUtil::setRandomDataProvider(0);
+    QUtil::setRandomDataProvider(nullptr);
     if (QUtil::getRandomDataProvider() != orig_rdp) {
         std::cout << "fail: passing null to setRandomDataProvider "
                      "didn't reset the random data provider\n";
diff --git a/libtests/rc4.cc b/libtests/rc4.cc
index 874c11ff..ca4af91d 100644
--- a/libtests/rc4.cc
+++ b/libtests/rc4.cc
@@ -51,7 +51,7 @@ main(int argc, char* argv[])
         t[1] = hexkey[i + 1];
         t[2] = '\0';
 
-        long val = strtol(t, 0, 16);
+        long val = strtol(t, nullptr, 16);
         key[i / 2] = static_cast<unsigned char>(val);
     }
 
diff --git a/qpdf/fix-qdf.cc b/qpdf/fix-qdf.cc
index 34b3d3dc..1d46971c 100644
--- a/qpdf/fix-qdf.cc
+++ b/qpdf/fix-qdf.cc
@@ -8,7 +8,7 @@
 #include <regex>
 #include <string_view>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 static void
 usage()
@@ -392,7 +392,7 @@ realmain(int argc, char* argv[])
 {
     whoami = QUtil::getWhoami(argv[0]);
     QUtil::setLineBuf(stdout);
-    char const* filename = 0;
+    char const* filename = nullptr;
     if (argc > 2) {
         usage();
     } else if ((argc > 1) && (strcmp(argv[1], "--version") == 0)) {
@@ -405,7 +405,7 @@ realmain(int argc, char* argv[])
         filename = argv[1];
     }
     std::string input;
-    if (filename == 0) {
+    if (filename == nullptr) {
         filename = "standard input";
         QUtil::binary_stdin();
         input = QUtil::read_file_into_string(stdin);
diff --git a/qpdf/pdf_from_scratch.cc b/qpdf/pdf_from_scratch.cc
index 1b172964..41e52f9c 100644
--- a/qpdf/pdf_from_scratch.cc
+++ b/qpdf/pdf_from_scratch.cc
@@ -9,7 +9,7 @@
 #include <cstdlib>
 #include <cstring>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 void
 usage()
@@ -86,7 +86,7 @@ int
 main(int argc, char* argv[])
 {
     QUtil::setLineBuf(stdout);
-    if ((whoami = strrchr(argv[0], '/')) == NULL) {
+    if ((whoami = strrchr(argv[0], '/')) == nullptr) {
         whoami = argv[0];
     } else {
         ++whoami;
diff --git a/qpdf/qpdf.cc b/qpdf/qpdf.cc
index 815a86b4..08e94d38 100644
--- a/qpdf/qpdf.cc
+++ b/qpdf/qpdf.cc
@@ -6,7 +6,7 @@
 #include <cstdlib>
 #include <iostream>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 static void
 usageExit(std::string const& msg)
diff --git a/qpdf/test_driver.cc b/qpdf/test_driver.cc
index 673c6b58..e9e7754c 100644
--- a/qpdf/test_driver.cc
+++ b/qpdf/test_driver.cc
@@ -37,7 +37,7 @@
 #define QPDF_OBJECT_NOWARN
 #include <qpdf/QPDFObject.hh>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 void
 usage()
@@ -286,7 +286,7 @@ test_0_1(QPDF& pdf, char const* arg2)
         qtest.pipeStreamData(out.get(), 0, qpdf_dl_none);
 
         std::cout << std::endl << "Uncompressed stream data:" << std::endl;
-        if (qtest.pipeStreamData(0, 0, qpdf_dl_all)) {
+        if (qtest.pipeStreamData(nullptr, 0, qpdf_dl_all)) {
             std::cout.flush();
             QUtil::binary_stdout();
             out = std::make_shared<Pl_StdioFile>("filtered", stdout);
@@ -387,7 +387,7 @@ test_4(QPDF& pdf, char const* arg2)
     }
 
     trailer.replaceKey("/Info", pdf.makeIndirectObject(qtest));
-    QPDFWriter w(pdf, 0);
+    QPDFWriter w(pdf, nullptr);
     w.setQDFMode(true);
     w.setStaticID(true);
     w.write();
@@ -599,7 +599,7 @@ test_12(QPDF& pdf, char const* arg2)
 # pragma GCC diagnostic push
 # pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 #endif
-    pdf.setOutputStreams(0, 0);
+    pdf.setOutputStreams(nullptr, nullptr);
 #if (defined(__GNUC__) || defined(__clang__))
 # pragma GCC diagnostic pop
 #endif
@@ -1009,7 +1009,7 @@ test_25(QPDF& pdf, char const* arg2)
     // Copy qtest without crossing page boundaries.  Should get O1
     // and O2 and their streams but not O3 or any other pages.
 
-    assert(arg2 != 0);
+    assert(arg2 != nullptr);
     {
         // Make sure original PDF is out of scope when we write.
         QPDF oldpdf;
@@ -1035,7 +1035,7 @@ test_26(QPDF& pdf, char const* arg2)
 
     {
         // Make sure original PDF is out of scope when we write.
-        assert(arg2 != 0);
+        assert(arg2 != nullptr);
         QPDF oldpdf;
         oldpdf.processFile(arg2);
         QPDFObjectHandle qtest = oldpdf.getTrailer().getKey("/QTest");
@@ -1104,7 +1104,7 @@ test_27(QPDF& pdf, char const* arg2)
         QPDFObjectHandle s3 = QPDFObjectHandle::newStream(&empty3);
         s3.replaceStreamData(
             p2, QPDFObjectHandle::newNull(), QPDFObjectHandle::newNull());
-        assert(arg2 != 0);
+        assert(arg2 != nullptr);
         QPDF oldpdf;
         oldpdf.processFile(arg2);
         QPDFObjectHandle qtest = oldpdf.getTrailer().getKey("/QTest");
@@ -1151,7 +1151,7 @@ static void
 test_29(QPDF& pdf, char const* arg2)
 {
     // Detect mixed objects in QPDFWriter
-    assert(arg2 != 0);
+    assert(arg2 != nullptr);
     auto other = QPDF::create();
     other->processFile(arg2);
     // We need to create a QPDF with mixed ownership to exercise
@@ -1201,7 +1201,7 @@ test_29(QPDF& pdf, char const* arg2)
 static void
 test_30(QPDF& pdf, char const* arg2)
 {
-    assert(arg2 != 0);
+    assert(arg2 != nullptr);
     QPDF encrypted;
     encrypted.processFile(arg2, "user");
     QPDFWriter w(pdf, "b.pdf");
@@ -1433,7 +1433,7 @@ test_40(QPDF& pdf, char const* arg2)
     // feature was implemented by Sahil Arora
     // <sahilarora.535@gmail.com> as part of a Google Summer of
     // Code project in 2017.
-    assert(arg2 != 0);
+    assert(arg2 != nullptr);
     QPDFWriter w(pdf, arg2);
     w.setPCLm(true);
     w.setStaticID(true);
@@ -3558,11 +3558,11 @@ runtest(int n, char const* filename1, char const* arg2)
 
     QPDF pdf;
     std::shared_ptr<char> file_buf;
-    FILE* filep = 0;
+    FILE* filep = nullptr;
     if (n == 0) {
         pdf.setAttemptRecovery(false);
     }
-    if (((n == 35) || (n == 36)) && (arg2 != 0)) {
+    if (((n == 35) || (n == 36)) && (arg2 != nullptr)) {
         // arg2 is password
         pdf.processFile(filename1, arg2);
     } else if (n == 45) {
@@ -3652,7 +3652,7 @@ int
 main(int argc, char* argv[])
 {
     QUtil::setLineBuf(stdout);
-    if ((whoami = strrchr(argv[0], '/')) == NULL) {
+    if ((whoami = strrchr(argv[0], '/')) == nullptr) {
         whoami = argv[0];
     } else {
         ++whoami;
diff --git a/qpdf/test_large_file.cc b/qpdf/test_large_file.cc
index b30967c7..aea7c0c5 100644
--- a/qpdf/test_large_file.cc
+++ b/qpdf/test_large_file.cc
@@ -35,7 +35,7 @@
 // reading the large file then allows us to verify large file support
 // with confidence.
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 // Height should be a multiple of 10
 static size_t const nstripes = 10;
@@ -47,7 +47,7 @@ static size_t const npages = 200;
 size_t stripesize = 0;
 size_t width = 0;
 size_t height = 0;
-static unsigned char* buf = 0;
+static unsigned char* buf = nullptr;
 
 static inline unsigned char
 get_pixel_color(size_t n, size_t row)
@@ -73,7 +73,7 @@ class ImageChecker: public Pipeline
 };
 
 ImageChecker::ImageChecker(size_t n) :
-    Pipeline("image checker", 0),
+    Pipeline("image checker", nullptr),
     n(n),
     offset(0),
     okay(true)
@@ -122,7 +122,7 @@ ImageProvider::ImageProvider(size_t n) :
 void
 ImageProvider::provideStreamData(int objid, int generation, Pipeline* pipeline)
 {
-    if (buf == 0) {
+    if (buf == nullptr) {
         buf = new unsigned char[width * stripesize];
     }
     std::cout << "page " << n << " of " << npages << std::endl;
diff --git a/qpdf/test_pdf_doc_encoding.cc b/qpdf/test_pdf_doc_encoding.cc
index cd2e9b42..e7aba030 100644
--- a/qpdf/test_pdf_doc_encoding.cc
+++ b/qpdf/test_pdf_doc_encoding.cc
@@ -4,7 +4,7 @@
 #include <cstdlib>
 #include <cstring>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 void
 usage()
@@ -16,7 +16,7 @@ usage()
 int
 main(int argc, char* argv[])
 {
-    if ((whoami = strrchr(argv[0], '/')) == NULL) {
+    if ((whoami = strrchr(argv[0], '/')) == nullptr) {
         whoami = argv[0];
     } else {
         ++whoami;
diff --git a/qpdf/test_pdf_unicode.cc b/qpdf/test_pdf_unicode.cc
index b28edeaf..325fcad8 100644
--- a/qpdf/test_pdf_unicode.cc
+++ b/qpdf/test_pdf_unicode.cc
@@ -4,7 +4,7 @@
 #include <cstdlib>
 #include <cstring>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 void
 usage()
@@ -16,7 +16,7 @@ usage()
 int
 main(int argc, char* argv[])
 {
-    if ((whoami = strrchr(argv[0], '/')) == NULL) {
+    if ((whoami = strrchr(argv[0], '/')) == nullptr) {
         whoami = argv[0];
     } else {
         ++whoami;
diff --git a/qpdf/test_tokenizer.cc b/qpdf/test_tokenizer.cc
index 8a58fc8e..d9c09c0b 100644
--- a/qpdf/test_tokenizer.cc
+++ b/qpdf/test_tokenizer.cc
@@ -11,7 +11,7 @@
 #include <cstdlib>
 #include <cstring>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 void
 usage()
@@ -91,7 +91,7 @@ tokenTypeName(QPDFTokenizer::token_type_e ttype)
     case QPDFTokenizer::tt_inline_image:
         return "inline-image";
     }
-    return 0;
+    return nullptr;
 }
 
 static std::string
@@ -238,13 +238,13 @@ int
 main(int argc, char* argv[])
 {
     QUtil::setLineBuf(stdout);
-    if ((whoami = strrchr(argv[0], '/')) == NULL) {
+    if ((whoami = strrchr(argv[0], '/')) == nullptr) {
         whoami = argv[0];
     } else {
         ++whoami;
     }
 
-    char const* filename = 0;
+    char const* filename = nullptr;
     size_t max_len = 0;
     bool include_ignorable = true;
     for (int i = 1; i < argc; ++i) {
@@ -265,7 +265,7 @@ main(int argc, char* argv[])
             filename = argv[i];
         }
     }
-    if (filename == 0) {
+    if (filename == nullptr) {
         usage();
     }
 
diff --git a/qpdf/test_unicode_filenames.cc b/qpdf/test_unicode_filenames.cc
index b2fd50f3..16478161 100644
--- a/qpdf/test_unicode_filenames.cc
+++ b/qpdf/test_unicode_filenames.cc
@@ -12,7 +12,7 @@
 static void
 do_copy(FILE* in, FILE* out)
 {
-    if ((in == 0) || (out == 0)) {
+    if ((in == nullptr) || (out == nullptr)) {
         std::cerr << "errors opening files" << std::endl;
         exit(2);
     }
diff --git a/zlib-flate/zlib-flate.cc b/zlib-flate/zlib-flate.cc
index 8f0f087a..d1311c84 100644
--- a/zlib-flate/zlib-flate.cc
+++ b/zlib-flate/zlib-flate.cc
@@ -9,7 +9,7 @@
 #include <cstdlib>
 #include <cstring>
 
-static char const* whoami = 0;
+static char const* whoami = nullptr;
 
 void
 usage()
@@ -27,7 +27,7 @@ usage()
 int
 main(int argc, char* argv[])
 {
-    if ((whoami = strrchr(argv[0], '/')) == NULL) {
+    if ((whoami = strrchr(argv[0], '/')) == nullptr) {
         whoami = argv[0];
     } else {
         ++whoami;