why does c++ std::min can't use a static field as its parameter when compile on O0?

问题

same code, compile with O0, it will report an error:

//============================================================================
// Name        : test.cpp
// Author      : 
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
#include <stdint.h>
using namespace std;
class foo{

      static const int64_t MAX_THREAD_NUM = 10 * 1000;


public:
      void test();
};

void foo::test(){
    int64_t a = 100;
//  int64_t tmp = MAX_THREAD_NUM;
//  int64_t min = std::min(tmp, a);


    int64_t min = std::min(MAX_THREAD_NUM, a);
    cout << min << endl; // prints !!!Hello World!!!

}

int main() {
    cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!
    return 0;
}

g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/test.d" -MT"src/test.o" -o "src/test.o" "../src/test.cpp"

g++  -o "test"  ./src/test.o   
./src/test.o: In function `foo::test()':
/home/foo/eclipse-workspace/test/Debug/../src/test.cpp:27: undefined reference to `foo::MAX_THREAD_NUM'
collect2: error: ld returned 1 exit status
/home/foo/eclipse-workspace/test/Debug/../src/test.cpp:27: undefined reference to `foo::MAX_THREAD_NUM'

but with O2 flag, it can compile succeed.

g++ -O2 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/test.d" -MT"src/test.o" -o "src/test.o" "../src/test.cpp"
g++  -o "test"  ./src/test.o

g++ version: g++ (Ubuntu 4.8.5-4ubuntu8) 4.8.5 Copyright (C) 2015 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE

回答1:

Program is ill-formed no diagnostic required (NDR) as you break One Definition Rule (ODR) by not providing definition of ODR-used symbol (std::min takes its argument by reference).

The optimizer removes that unused code and lets you think it is correct.

回答2:

As pointed out by @dfri, compiler inlines MAX_THREAD_NUM. If we change it to inline static const int64_t MAX_THREAD_NUM = 10 * 1000;, it will compile fine on O0. Otherwise, declare but define the variable outside of the class, or mark it constexpr instead of const. Using a static const integral member needs definition if it's used where a non-const integral is needed.

class foo {
      static const int64_t MAX_THREAD_NUM;
      ...
};
const int64_t foo::MAX_THREAD_NUM = 10 * 1000;

来源：https://stackoverflow.com/questions/63575194/why-does-c-stdmin-cant-use-a-static-field-as-its-parameter-when-compile-on

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