c – 概念和申报单

#include <iostream>
#include <string>

// Uncomment this declaration to change behavIoUr
//void draw(const std::string&);

template <typename T>
concept bool Drawable() {
    return requires (const T& t) {
        { draw(t) }
    };
}

void draw(const std::string& s)
{
    std::cout << s << "\n";
}

int main()
{
    static_assert(Drawable<std::string>()); // Fails
}

这个问题与ADL毫无关系),只是通过名称查找. GCC使用的概念草案是n4377,但是我将使用的C标准草案是n4140.首先,在潜入标准之前,我们可以将您的问题变成我们知道应该运作的形式的MCVE.例：

template<typename T> concept bool C =
  requires (T a,T b) {
    a + b;
  };

这是一个简单的要求,[expr.prim.req.simple],它检查表达式的有效性.重写我们的示例以匹配表单：

template<typename T> concept bool Drawable = 
  requires (const T& x) { 
    draw(x); 
  };

我们可以看到我们的语法很好.好的,n4377怎么说？

[expr.prim.req]/1 A requires-expression provides a concise way to
express requirements on template arguments. A requirement is one that
can be checked by name lookup (3.4) or by checking properties of types
and expressions.

[expr.prim.req]/6 The requirement-body is comprised of a sequence of
requirements. These requirements may refer to local parameters,
template parameters,and any other declarations visible from the
enclosing context. …

说得通.我们知道包围的上下文是全局命名空间,所以n4140说什么？

[basic.lookup.unqual]/1 In all the cases listed in 3.4.1,the scopes
are searched for a declaration in the order listed in each of the
respective categories; name lookup ends as soon as a declaration is
found for the name. If no declaration is found,the program is
ill-formed.

A name used in the deFinition of a function following the function’s
declarator-id that is a member of namespace N (where,only for the purpose of exposition,N Could represent the global scope) shall be
declared before its use in the block in which it is used or in one of
its enclosing blocks (6.3) or,shall be declared before its use in
namespace N …

由于这个概念适用于该功能,所以上面的段落适用.