DBL_MAX的类型 [英] Type of DBL_MAX

问题描述

我在某处看到了这段代码

I saw this code somewhere

printf("DBL_MAX     :   %g\n", (double) DBL_MAX);

演员阵容是否必要?我找不到标准中指定DBL_MAX实际上必须具有(或提升为)double类型的任何内容.

Is the cast necessary? I cannot find anything in the standard which specifies DBL_MAX must actually have (or promote to) type double.

推荐答案

DBL_MAX被指定为宏，因此它本身没有类型.

DBL_MAX is specified to be a macro, so in itself, does not have a type.

1999 C标准的要求是DBL_MAX必须扩展为至少为1E+37的恒定值，但是没有提及其实际类型.由于它表示double可以表示的实现定义的最大值，因此逻辑将建议扩展是可以直接转换为double的值，而无需进行任何更改(例如，精度损失).

The requirement from the 1999 C standard is that DBL_MAX must expand to a constant value that is at least 1E+37, but nothing is said about its actual type. Since it represents the implementation-defined maximum value that a double can represent, logic would suggest the expansion is a value that can be converted to a double without any change (e.g. loss of precision).

此逻辑将允许但不要求DBL_MAX的类型为double.它也可以是long double类型(因为还要求long double能够准确表示double可以的所有值).

This logic would allow but not require DBL_MAX to be of type double. It could also be of type long double (since it is also required that a long double can exactly represent all values that a double can).

实际上，很难设想将DBL_MAX扩展为非double类型的常量的实现，而且我当然从没听说过一个实现.但这并不意味着它是必需的. printf("DBL_MAX : %g\n", (double) DBL_MAX)提供一种保守的保证(就可能发生的可能性而言，即为可能的可能性进行了规定)，保证将类型double的值传递给printf().

Practically, it is difficult to envisage an implementation which has DBL_MAX expand to a constant that is not of type double, and I certainly have never heard mention of one. But that does not mean it is required. printf("DBL_MAX : %g\n", (double) DBL_MAX) provides a conservative (in the sense of making provision for a possibility that is allowed, even if it might not occur) guarantee that a value of type double is passed to printf().

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