c++ - 相同的 const 类型限定符

Question

一个类型有两个 const 会发出警告/错误。但是，如果类型已使用 typedef 定义，编译器会接受它(Visual Studio 2013 和在线编译器 C++ shell)。

#include <iostream>

typedef const int value_type;

int main()
{
    const value_type  n = 0;   //ok 
    const const int   n2 = 0;  //error C4114

    return 0;
}

有人知道为什么吗？是不是那个是const (const int)，和const const int不一样？

Answer 1

在 typedef 情况下明确允许，而在声明本身中不允许:

[dcl.type/1]

The type-specifiers are:

type-specifier : ... cv-qualifier
defining-type-specifier : type-specifier

[dcl.spec/1 and 2]

The specifiers that can be used in a declaration are:

decl-specifier : ... defining-type-specifier ...

Each decl-specifier shall appear at most once in a complete decl-specifier-seq, except that long may appear twice.

[dcl.type.cv/1]

There are two cv-qualifiers, const and volatile. Each cv-qualifier shall appear at most once in a cv-qualifier-seq. If a cv-qualifier appears in a decl-specifier-seq, the init-declarator-list or member-declarator-list of the declaration shall not be empty. [ Note: [basic.type.qualifier] and [dcl.fct] describe how cv-qualifiers affect object and function types.  — end note ] Redundant cv-qualifications are ignored. [ Note: For example, these could be introduced by typedefs. — end note ]

除了类型别名之外，模板参数是限定符可能是多余的另一种情况。允许这样做的理由是不要仅仅因为 cv 限定符偷偷进入后门就破坏其他正确的声明。