c++ - c++11 get string length in compile time by constexpr -

#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 = constlength(p);     printf(p);     printf("%d", i);     return 0; } 

hi,all want length of string in compile-time.so wrote code above.but in disassembly code found 'constlength' function named sub_401000 below lead run-time overhead computting length of string.is there wrong?(visual studio 2015 preview,release 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; } 

a constexpr function can evaluated @ compile time when called arguments compile-time constants. although value of p can determined static analysis (it doesn't change between initialization , evaluation), it's not constant expression per standard definition.

try this:

constexpr const char* p = "1234567"; 

also, can guarantee initialization doable without runtime overhead declaring initialized variable constexpr:

constexpr size_t = constlength(p);