Add --force-empty-vectors option (#5653)

The rationale for this option is that JSON clients typically want empty arrays (i.e [] in the JSON) instead of missing properties, but not empty strings when the value isn't set.
--force-empty is kept as-is, i.e. it will force both empty strings and vectors.

Closes #5652

docs/source/Compiler.md

@@ -203,5 +203,8 @@ Additional options:
 -   `--force-empty` : When serializing from object API representation, force
      strings and vectors to empty rather than null.
 
+-   `--force-empty-vectors` : When serializing from object API representation, force
+     vectors to empty rather than null.
+
 NOTE: short-form options for generators are deprecated, use the long form
 whenever possible.

include/flatbuffers/idl.h

@@ -588,9 +588,13 @@ struct IDLOptions {
   // for code generation.
   unsigned long lang_to_generate;
 
-  // If set (default behavior), empty string and vector fields will be set to
+  // If set (default behavior), empty string fields will be set to nullptr to make
-  // nullptr to make the flatbuffer more compact.
+  // the flatbuffer more compact.
-  bool set_empty_to_null;
+  bool set_empty_strings_to_null;
+
+  // If set (default behavior), empty vector fields will be set to nullptr to make
+  // the flatbuffer more compact.
+  bool set_empty_vectors_to_null;
 
   IDLOptions()
       : use_flexbuffers(false),
@@ -635,7 +639,8 @@ struct IDLOptions {
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
         lang_to_generate(0),
-        set_empty_to_null(true) {}
+        set_empty_strings_to_null(true),
+        set_empty_vectors_to_null(true) {}
 };
 
 // This encapsulates where the parser is in the current source file.

src/flatc.cpp

@@ -63,7 +63,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     const Generator &g = params_.generators[i];
 
     std::stringstream full_name;
-    full_name << std::setw(12) << std::left << g.generator_opt_long;
+    full_name << std::setw(16) << std::left << g.generator_opt_long;
     const char *name = g.generator_opt_short ? g.generator_opt_short : "  ";
     const char *help = g.generator_help;
 
@@ -101,7 +101,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --gen-object-api       Generate an additional object-based API.\n"
     "  --gen-compare          Generate operator== for object-based API types.\n"
     "  --gen-nullable         Add Clang _Nullable for C++ pointer. or @Nullable for Java\n"
-    "  --java-checkerframework Add @Pure for Java.\n"
+    "  --java-checkerframe    work Add @Pure for Java.\n"
     "  --gen-generated        Add @Generated annotation for Java\n"
     "  --gen-all              Generate not just code for the current schema files,\n"
     "                         but for all files it includes as well.\n"
@@ -149,6 +149,8 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --force-defaults       Emit default values in binary output from JSON\n"
     "  --force-empty          When serializing from object API representation,\n"
     "                         force strings and vectors to empty rather than null.\n"
+    "  --force-empty-vectors  When serializing from object API representation,\n"
+    "                         force vectors to empty rather than null.\n"
     "  --flexbuffers          Used with \"binary\" and \"json\" options, it generates\n"
     "                         data using schema-less FlexBuffers.\n"
     "FILEs may be schemas (must end in .fbs), binary schemas (must end in .bfbs),\n"
@@ -323,7 +325,10 @@ int FlatCompiler::Compile(int argc, const char **argv) {
       } else if (arg == "--force-defaults") {
         opts.force_defaults = true;
       } else if (arg == "--force-empty") {
-        opts.set_empty_to_null = false;
+        opts.set_empty_strings_to_null = false;
+        opts.set_empty_vectors_to_null = false;
+      } else if (arg == "--force-empty-vectors") {
+        opts.set_empty_vectors_to_null = false;
       } else if (arg == "--java-primitive-has-method") {
         opts.java_primitive_has_method = true;
       } else if (arg == "--flexbuffers") {

src/idl_gen_cpp.cpp

@@ -2452,10 +2452,10 @@ class CppGenerator : public BaseGenerator {
 
         // For optional fields, check to see if there actually is any data
         // in _o->field before attempting to access it. If there isn't,
-        // depending on set_empty_to_null either set it to 0 or an empty string.
+        // depending on set_empty_strings_to_null either set it to 0 or an empty string.
         if (!field.required) {
           auto empty_value =
-              opts.set_empty_to_null ? "0" : "_fbb.CreateSharedString(\"\")";
+              opts.set_empty_strings_to_null ? "0" : "_fbb.CreateSharedString(\"\")";
           code = value + ".empty() ? " + empty_value + " : " + code;
         }
         break;
@@ -2557,10 +2557,10 @@ class CppGenerator : public BaseGenerator {
           }
         }
 
-        // If set_empty_to_null option is enabled, for optional fields, check to
+        // If set_empty_vectors_to_null option is enabled, for optional fields, check to
         // see if there actually is any data in _o->field before attempting to
         // access it.
-        if (opts.set_empty_to_null && !field.required) {
+        if (opts.set_empty_vectors_to_null && !field.required) {
           code = value + ".size() ? " + code + " : 0";
         }
         break;