Infix vs前缀语法：名称查找差异 [英] Infix vs prefix syntax: name lookup differences
问题描述
C ++中的运算符通常被认为是函数/方法的替代语法，特别是在重载的上下文中。如果是这样，下面的两个表达式应该是同义词：
Operators in C++ are usually considered to be an alternative syntax for functions/methods, especially in the context of overloading. If so, the two expressions below should be synonymous:
std::cout << 42;
operator<<(std::cout, 42);
实际上，第二个语句会导致以下错误： p>
In practise, the second statement leads to the following error:
call of overloaded ‘operator<<(std::ostream&, int)’ is ambiguous
像往常一样，此类错误消息附有可能的候选列表，其中包括：
As usual, such error message is accompanied with a list of possible candidates, these are:
operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
operator<<(basic_ostream<char, _Traits>& __out, char __c)
operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
此错误至少会产生两个问题：
Such error raises at least two questions:
- 为什么
运算符<<（basic_ostream< char ，_Traits>& __out，int__ c）缺少？
- In what way are the two statements different (in terms of name lookup)?
- Why
operator<<(basic_ostream<char, _Traits>& __out,int__c)is missing?
看来，中缀和前缀符号不能完全互换 - 不同的语法需要不同的名称解析策略。
It seems, that infix and prefix notations are not fully interchangeable -- different syntax entails different name resolution tactics. What are the differences and where did they come from?
推荐答案
不，这两个表达式不应该是同义词。 std :: cout<< 42 被查找为运算符<<<（std :: cout，42）和 std :: cout。 operator<<（42）。这两个查找产生可行的候选人，但第二个是更好的匹配。
No, the two expressions should not be synonymous. std::cout << 42 is looked up as both operator<<(std::cout, 42) and std::cout.operator<<(42). Both lookups produce viable candidates, but the second one is a better match.
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