从std :: function到bool没有可行的转换 [英] No viable conversion from std::function to bool

问题描述

C ++ 11 std :: function 应该实现 operator bool（）const ，为什么clang告诉我没有可行的转换？

  #include< functional> 
 #include< cstdio> 
 
 inline double the_answer（）
 {return 42.0; } 
 
 int main（）
 {
 std :: function< double（）> F; 
 
 bool yes =（f = the_answer）; 
 
 if（yes）printf（答案是％.2f \\\
，f（））; 
} 
  

编译错误是：

  function_bool.cpp：12：7：错误：没有可行的转换从'std :: function< double（）>'到'bool'
 bool yes = （f = the_answer）; 
 ^ ~~~~~~~~~~~~~~~~ 
 1错误生成。 
  

EDIT 我没有看到 关键字..没有隐式转换，我想我必须使用 static_cast 。

c> c> c> c> code> explicit ，因此它不能用于复制初始化。你可以直接初始化：

  bool yes（f = the_answer）; 
  

但是，我假设它真的用于上下文转换表达式用作条件，最常用于 if 语句。与隐式转换不同，上下文转换可以调用显式构造函数和转换函数。

  //这很好（虽然编译器可能警告）
 if（f = the_answer）{
 // ... 
} 
  pre> 
The C++11 std::function is supposed to implement operator bool() const, so why does clang tell me there is no viable conversion?
#include <functional>
#include <cstdio>

inline double the_answer() 
    { return 42.0; }

int main()
{
    std::function<double()> f;

    bool yes = (f = the_answer);

    if (yes) printf("The answer is %.2f\n",f());
}
The compiling error is:
function_bool.cpp:12:7: error: no viable conversion from 'std::function<double ()>' to 'bool'
        bool yes = (f = the_answer);
             ^     ~~~~~~~~~~~~~~~~
1 error generated.
EDIT I didn't see the explicit keyword.. no implicit conversion then, I guess I'll have to use static_cast.
解决方案
operator bool() for std::function is explicit, therefore it cannot be used for copy-initialization. You can actually do direct-initialization:
bool yes(f = the_answer);
However, I assume it's really intended for contextual conversion, which happens when an expression is used as a condition, most often for an if statement. Contextual conversion can call explicit constructors and conversion functions, unlike implicit conversion.
// this is fine (although compiler might warn)
if (f = the_answer) {
    // ...
}


                        
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