爲什麼必須在運行時構建一個字符串？

Question

可以將C-Strings或std::string創建爲constexpr還是必須在運行時創建？

用gcc 4.9.2我可以這樣做：

constexpr const char foo[] = "blee"; 

（可悲的是2013年11月客戶技術預覽不允許Visual Studio來支持這一點：https://stackoverflow.com/a/29255013/2642059）

但即便用gcc 4.9。 2我不能做到這一點：

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

我得到的錯誤：

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 

但我想對進行更多的說明，爲什麼 a std::string不是文字。也就是說：爲什麼必須在運行時構建一個字符串？

正如指出的這個問題可以部分回答這個：Is it possible to use std::string in a constexpr?但它並沒有涉及爲什麼std::string不能成爲問題的核心文字。

Answer 1

有一個constexpr字符串的提議：Compile-Time String: std::string_literal和它說：

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

這也印證確實std::string不是文本類型。

那麼爲什麼不讓std::string成爲文字類型呢？

我們得到一個提示，爲何從上面爲什麼這是不可能的建議：

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需要動態內存是無法在編譯時。

爲什麼constexpr不能被應用到的std :: string但可以爲CHAR

constexpr應用於對象應被施加到文本類型的陣列，它並不適用於std::string但適用於一組const char。從C++ 11標準牽伸部7.1.5[dcl.constexpr]（重點礦山前進）：

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). […]

和從部分3.9[鹼性。類型]：

A type is a literal type if it is:

，幷包括：

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

運算類型是標量類型，幷包括炭，它覆蓋的const char

的陣列和用於類：

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不符合該標準。

Answer 2

不能使用constexpr，因爲std :: string沒有小事destructor。檢查cppreference的要求。