#include与#define位置的相对关系可能会影响程序的运行

例如

#include<iostream>
#define NDEBUG
#include<cassert>
// #define NDEBUG
int main(){
    assert(0);
    std::cout << "hello world!" << std::endl;
    return 0;
}

运行后，输出 hello world!
而下段代码运行后

#include<iostream>
#define NDEBUG
#include<cassert>
// #define NDEBUG
int main(){
    assert(0);
    std::cout << "hello world!" << std::endl;
    return 0;
}

assert输出信息，终止程序运行。
究其原因，是#include<cassert>与#define NDEBUG均在预处理过程中展开，观察cassert的源代码。

/*
 * assert.h
 * This file has no copyright assigned and is placed in the Public Domain.
 * This file is a part of the mingw-runtime package.
 * No warranty is given; refer to the file DISCLAIMER within the package.
 *
 * Define the assert macro for debug output.
 *
 */

/* We should be able to include this file multiple times to allow the assert
   macro to be enabled/disabled for different parts of code.  So don't add a
   header guard.  */

#ifndef RC_INVOKED

/* All the headers include this file. */
#include <_mingw.h>

#undef assert

#ifdef  __cplusplus
extern "C" {
#endif

#ifdef NDEBUG
/*
 * If not debugging, assert does nothing.
 */
#define assert(x)   ((void)0)

#else /* debugging enabled */

/*
 * CRTDLL nicely supplies a function which does the actual output and
 * call to abort.
 */
_CRTIMP void __cdecl __MINGW_NOTHROW _assert (const char*, const char*, int) __MINGW_ATTRIB_NORETURN;

/*
 * Definition of the assert macro.
 */
#define assert(e)       ((e) ? (void)0 : _assert(#e, __FILE__, __LINE__))

#endif  /* NDEBUG */

#ifdef  __cplusplus
}
#endif

#endif /* Not RC_INVOKED */

预处理器实际是将头文件的内容插入当前文件位置。所以#define NDEBUG与#include<cassert>的相对位置会影响程序运行结果。而在编译命令中使用 -D NDEBUG则不会产生该情况。