C++ no equal sign but compiles without assigning value - c++

Question

While tracking down a bug in my software, I encountered a weird behavior, and was wondering if anyone would have some hindsight.
Here is the code (it compiles and does not have any runtime errors):
#define ERR_OK 0
#define ERR_NOT_OK -41
int some_class::some_function(uint32_t verbose){
int ret_value = ERR_OK;
if (some_condition) {
ret_value ERR_NOT_OK;
}
return ret_value;
}
Why does this line compile; it's missing an equal sign:
ret_value ERR_NOT_OK;
Xcodebuild log:
CompileC /Users/user_name/Library/Developer/Xcode/DerivedData/xxx-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/Objects-normal/x86_64/some_class.o api/src/some_class.cpp normal x86_64 c++ com.apple.compilers.llvm.clang.1_0.compiler
cd /Users/user_name/Documents/project_name/project_name_dev
export LANG=en_US.US-ASCII
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -x c++ -arch x86_64 -fmessage-length=0 -fdiagnostics-show-note-include-stack -fmacro-backtrace-limit=0 -Wno-trigraphs -fpascal-strings -O0 -fno-common -Werror -Wno-missing-field-initializers -Wno-missing-prototypes -Wunreachable-code -Wno-non-virtual-dtor -Wno-overloaded-virtual -Wno-exit-time-destructors -Wno-missing-braces -Wparentheses -Wswitch -Wunused-function -Wno-unused-label -Wno-unused-parameter -Wunused-variable -Wunused-value -Wempty-body -Wuninitialized -Wno-unknown-pragmas -Wno-shadow -Wno-four-char-constants -Wno-conversion -Wconstant-conversion -Wint-conversion -Wbool-conversion -Wenum-conversion -Wassign-enum -Wshorten-64-to-32 -Wno-newline-eof -Wno-c++11-extensions -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -fasm-blocks -funroll-loops -fstrict-aliasing -Wno-deprecated-declarations -Winvalid-offsetof -mmacosx-version-min=10.10 -g -Wno-sign-conversion -Winfinite-recursion -Wmove -iquote /Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/project_nameAPI-generated-files.hmap -I/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/project_nameAPI-own-target-headers.hmap -I/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/project_nameAPI-all-target-headers.hmap -iquote /Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/project_nameAPI-project-headers.hmap -I/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Products/Debug/include -I/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/DerivedSources/include -I/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/DerivedSources/x86_64 -I/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/DerivedSources -Wno-address-of-packed-member -Wno-unknown-warning-option -F/Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Products/Debug -Wall -MMD -MT dependencies -MF /Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/Objects-normal/x86_64/some_class.d --serialize-diagnostics /Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/Objects-normal/x86_64/some_class.dia -c /Users/user_name/Documents/project_name/project_name_dev/api/src/some_class.cpp -o /Users/user_name/Library/Developer/Xcode/DerivedData/jal-emkfpcfmbsgbyrammutcuwrgpqox/Build/Intermediates/project_name.build/Debug/project_nameAPI.build/Objects-normal/x86_64/some_class.o
Edit: Removed semicolons and changed #define ERR_NOT_OK 1 to #define ERR_NOT_OK -41; added xcodebuild log.

Answer 1

If ERR_NOT_OK is defined as -41, then your
ret_value ERR_NOT_OK;
is substituted with
ret_value -41;
which is a valid expression statement, even though it is effectively a no-op. What was originally intended as unary - gets interpreted as binary - in this context.
This is why it is a good idea to define it as
#define ERR_NOT_OK (-41)