什么时候应用ADL? [英] When is ADL applied?
问题描述
有3个示例:
我。
typedef int foo;
namespace B
{
struct S
{
operator int(){ return 24; }
};
int foo(B::S s){ return 0; }
}
int main()
{
int t=foo(B::S()); //24, ADL does not apply
}
II。
namespace B
{
struct S
{
operator int(){ return 24; }
};
int foo(B::S s){ return 0; }
}
int main()
{
int t=foo(B::S()); //0, ADL applies
}
III。
namespace B
{
struct S
{
operator int(){ return 24; }
};
int foo(B::S s){ return 0; }
}
int foo(B::S s){ return 12; }
int main()
{
int t=foo(B::S()); //error: call of overloaded ‘foo(B::S)’ is ambiguous
//ADL applies
}
对我来说不清楚什么是ADL查找的实际条件将适用?
It is not clear for me what is the actual conditions to ADL lookup will be apply? I need in reference to standard described it.
推荐答案
这个标准段落阐明了,甚至有一个非常类似于你的第一个例子的例子。
This Standard paragraph clarifies, and even has an example very much like your first example.
3.4.1 / 3:
3.4.1/3:
查找未限定名称用作函数调用的 postfix-expression ,如3.4.2 [basic.lookup.argdep]中所述。 [注意:为了确定(在解析期间)表达式是否为函数调用的 postfix-expression ,通常使用名称查找规则。 3.4.2中的规则对表达式的句法解释没有影响。例如,
The lookup for an unqualified name used as the postfix-expression of a function call is described in 3.4.2 [basic.lookup.argdep]. [Note: For purposes of determining (during parsing) whether an expression is a postfix-expression for a function call, the usual name lookup rules apply. The rules in 3.4.2 have no effect on the syntactic interpretation of an expression. For example,
typedef int f;
namespace N {
struct A {
friend void f(A &);
operator int();
void g(A a) {
int i = f(a); // f is the typedef, not the friend
// function: equivalent to int(a)
}
};
}
因为表达式不是函数调用,参数相关的名称查找(3.4.2)不适用,并且未找到朋友函数
f
。 - 结束注释]
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