constexpr问题，为什么这两个不同的程序运行在不同的时间与g ++？ [英] constexpr question, why do these two different programs run in such a different amount of time with g++?

问题描述

我使用gcc 4.6.1，并得到一些有趣的行为涉及调用 constexpr 函数。这个程序运行很好，直接打印输出 12200160415121876738 。

  include< iostream> 
 
 extern const unsigned long joe; 
 
 constexpr unsigned long fib（unsigned long int x）
 {
 return（x< = 1）？ 1：（fib（x-1）+ fib（x-2））; 
} 
 
 const unsigned long joe = fib（92）; 
 
 int main（）
 {
 :: std :: cout< 这里我是！\\\
; 
 :: std :: cout<< joe<< '\\\
'; 
 return 0; 
} 
  

这个程序永远运行，我从来没有耐心等待以便打印出一个值：

  #include< iostream> 
 
 constexpr unsigned long fib（unsigned long int x）
 {
 return（x< = 1）？ 1：（fib（x-1）+ fib（x-2））; 
} 
 
 int main（）
 {
 :: std :: cout< 这里我是！\\\
; 
 :: std :: cout<<纤维（92） '\\\
'; 
 return 0; 
} 
  

为什么会有这么大的差别？

编辑： g ++ -std =

解决方案

joe 是一个积分常数表达式;它必须可用于数组边界。因此，合理的编译器将在编译时对其进行评估。

在第二个程序中，即使编译器可以在编译时计算它，必须。

I'm using gcc 4.6.1 and am getting some interesting behavior involving calling a constexpr function. This program runs just fine and straight away prints out 12200160415121876738.

#include <iostream>

extern const unsigned long joe;

constexpr unsigned long fib(unsigned long int x)
{
   return (x <= 1) ? 1 : (fib(x - 1) + fib(x - 2));
}

const unsigned long joe = fib(92);

int main()
{
   ::std::cout << "Here I am!\n";
   ::std::cout << joe << '\n';
   return 0;
}

This program takes forever to run and I've never had the patience to wait for it to print out a value:

#include <iostream>

constexpr unsigned long fib(unsigned long int x)
{
   return (x <= 1) ? 1 : (fib(x - 1) + fib(x - 2));
}

int main()
{
   ::std::cout << "Here I am!\n";
   ::std::cout << fib(92) << '\n';
   return 0;
}

Why is there such a huge difference? Am I doing something wrong in the second program?

Edit: I'm compiling this with g++ -std=c++0x -O3 on a 64-bit platform.

解决方案

joe is an Integral Constant Expression; it must be usable in array bounds. For that reason, a reasonable compiler will evaluate it at compile time.

In your second program, even though the compiler could calculate it at compile time, there's no reason why it must.

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