常量C表达式是在编译时还是在运行时评估的？(Are constant C expressions evaluated at compile time or at runtime?)

 如果我编写一个使用其他预处理器常量执行操作的#define ，每次宏在运行时出现时计算的最终值是多少？ 这是否取决于编译器中的优化，还是包含在标准之下？ 
 
 例： 
 
#define EXTERNAL_CLOCK_FREQUENCY    32768
#define TIMER_1_S                   EXTERNAL_CLOCK_FREQUENCY
#define TIMER_100_MS                TIMERB_1_S / 10
 
 每当我使用TIMER_100_MS宏时，操作32768/10会在运行时发生吗？ 
 
 我想避免以下情况： 
 
#define EXTERNAL_CLOCK_FREQUENCY    32768
#define TIMER_1_S                   EXTERNAL_CLOCK_FREQUENCY
#define TIMER_100_MS                3276
 
 
 概要 
 
 编译器需要能够计算常量积分表达式，因为它们在编译时需要用于计算诸如数组大小之类的东西。 然而，标准只是说他们“可以” - 而不是“必须” - 这样做。 因此，只有脑死亡编译器才会在编译时计算常量积分表达式，但是对非常规编译器的汇编输出进行简单检查将验证每种情况。 

If I write a #define that performs an operation using other preprocessor constants, is the final value computed each time the macro appears at runtime? Does this depend on optimizations in the compiler, or is it covered under a standard?
 
Example:
 
#define EXTERNAL_CLOCK_FREQUENCY    32768
#define TIMER_1_S                   EXTERNAL_CLOCK_FREQUENCY
#define TIMER_100_MS                TIMERB_1_S / 10
 
Will the operation 32768 / 10 occur at runtime every time I use the TIMER_100_MS macro?
 
I would like to avoid the following:
 
#define EXTERNAL_CLOCK_FREQUENCY    32768
#define TIMER_1_S                   EXTERNAL_CLOCK_FREQUENCY
#define TIMER_100_MS                3276
 
 
Summary
 
A compiler is required to be able to evaluate constant integral expressions because they are necessary for calculating things like array sizes at compile time. However, the standard only says they "can" -- not "must" -- do so. Therefore, only a brain-dead compiler would not evaluate a constant integral expressions at compile time, but a simple check of the assembly output for an unconventional compiler would verify each case.

原文：https://stackoverflow.com/questions/436300