c ++ 11通过constexpr在编译时获取字符串长度 [英] c++11 get string length in compile time by constexpr

问题描述

#include <stdio.h>

constexpr size_t constLength(const char* str)
{
    return (*str == 0) ? 0 : constLength(str + 1) + 1;
}

int _tmain(int argc, _TCHAR* argv[])
{   
    const char* p = "1234567";
    size_t i = constLength(p);
    printf(p);
    printf("%d", i);
    return 0;
}

您好，所有
我都想获取字符串的长度因此，我在上面编写了代码。但是在反汇编代码中，我发现下面的名为sub_401000的'constLength'函数将导致计算字符串长度的运行时开销。是否有问题？（Visual Studio 2015 Preview，释放并以最大化速度（/ O2）优化）

Hi,all I want to get the length of a string in compile-time.So i wrote the code above.But in disassembly code i found 'constLength' function that named sub_401000 below will lead run-time overhead for computting the length of string.Is there something wrong?(Visual Studio 2015 Preview,release with Maximize Speed (/O2) optimization)

int __cdecl sub_401010()
{
    int v0; // esi@1

    v0 = sub_401000("234567") + 1;
    sub_401040(&unk_402130);
    sub_401040("%d");
     return 0;
}

int __thiscall sub_401000(void *this)
{
  int result; // eax@2

  if ( *(_BYTE *)this )
    result = sub_401000((char *)this + 1) + 1;
  else
    result = 0;
  return result;
}

推荐答案

A constexpr 函数只能在使用编译时常量的参数调用时在编译时求值。尽管 p 的值可以通过静态分析确定（在初始化和评估之间不会改变），但根据标准定义，它不是常数表达式。

A constexpr function can only be evaluated at compile time when called with arguments that are compile-time constants. Although the value of p can be determined by static analysis (it doesn't change between initialization and evaluation), it's not a constant expression per the standard definition.

尝试一下：

constexpr const char* p = "1234567";

另外，您可以通过将初始化变量声明为<$来保证初始化是可行的，而不会增加运行时的开销c $ c> constexpr ：

Also, you can guarantee that an initialization is doable without runtime overhead by declaring the initialized variable as constexpr:

constexpr size_t i = constLength(p);

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