为什么必须在运行时构建一个字符串？

Question

可以将C-Strings或std::string创建为constexpr还是必须在运行时创建？

用gcc 4.9.2我可以这样做：

constexpr const char foo[] = "blee"; 

（可悲的是2013年11月客户技术预览不允许Visual Studio来支持这一点：https://stackoverflow.com/a/29255013/2642059）

但即便用gcc 4.9。 2我不能做到这一点：

constexpr const std::string foo = "blee"; 

我得到的错误：

error: the type 'const string {aka const std::basic_string<char>}' of constexpr variable 'foo' 
     is not literal 

constexpr const std::string foo = "blee"; 
           ^
note: 'std::basic_string<char>' is not literal because: 
    class basic_string 
     ^
note: 'std::basic_string<char>' has a non-trivial destructor 

但我想对进行更多的说明，为什么 a std::string不是文字。也就是说：为什么必须在运行时构建一个字符串？

正如指出的这个问题可以部分回答这个：Is it possible to use std::string in a constexpr?但它并没有涉及为什么std::string不能成为问题的核心文字。

Answer 1

有一个constexpr字符串的提议：Compile-Time String: std::string_literal和它说：

The purpose of std::string_literal , like std::string , is to provide a convenience utility for working with text. Unlike std::string , an instantiation of std::string_literal is a literal type and so can be used at compile­time. That is, it may be the type of an constexpr object, and it may be the type of a parameter, return value or local variable of a constexpr function

这也印证确实std::string不是文本类型。

那么为什么不让std::string成为文字类型呢？

我们得到一个提示，为何从上面为什么这是不可能的建议：

This would require a massive core language change to make something like dynamic memory available at compile­-time, or to make something like VLA/ARB and permit them in literal types. Given the violently negative reaction of Rapperswil Evolution to not only N4025 (Classes of Runtime Size), but anything that vaguely resembles VLA/ARBs, we can expect this not to happen any time soon, so this idea is a non­starter.

std::string需要动态内存是无法在编译时。

为什么constexpr不能被应用到的std :: string但可以为CHAR

constexpr应用于对象应被施加到文本类型的阵列，它并不适用于std::string但适用于一组const char。从C++ 11标准牵伸部7.1.5[dcl.constexpr]（重点矿山前进）：

A constexpr specifier used in an object declaration declares the object as const . Such an object shall have literal type and shall be initialized. If it is initialized by a constructor call, that call shall be a constant expression (5.19). […]

和从部分3.9[碱性。类型]：

A type is a literal type if it is:

，并包括：

• a scalar type; or
• an array of literal type

运算类型是标量类型，并包括炭，它覆盖的const char

的阵列和用于类：

a class type (Clause 9) that has all of the following properties:

• it has a trivial destructor,
• every constructor call and full-expression in the brace-or-equal-initializers for non-static data members (if any) is a constant expression (5.19),
• it is an aggregate type (8.5.1) or has at least one constexpr constructor or constructor template that is not a copy or move constructor, and
• all of its non- static data members and base classes are of literal types.

std::string不符合该标准。

Answer 2

不能使用constexpr，因为std :: string没有小事destructor。检查cppreference的要求。