C++ 沒有等號但編譯時不賦值

Question

在追蹤我的軟件中的錯誤時，我遇到了一個奇怪的行為，想知道是否有人會有事后的看法。

這是代碼（它可以編譯並且沒有任何運行時錯誤）：

#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;
}

為什么這一行編譯； 它缺少一個等號：

ret_value ERR_NOT_OK;

Xcode構建日志：

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

編輯：刪除分號並將#define ERR_NOT_OK 1 更改為#define ERR_NOT_OK -41； 添加了 xcodebuild 日志。

Answer 1

如果ERR_NOT_OK被定義為-41 ，那么你的

ret_value ERR_NOT_OK;

被替換為

ret_value -41;

這是一個有效的表達式語句，即使它實際上是一個空操作。 什么本來是打算作為一元-被解釋為二進制-在這種情況下。

這就是為什么將其定義為

#define ERR_NOT_OK (-41)