問題描述
下面函數(shù)返回的指針是否有效?
Is the pointer returned by the following function valid?
const char * bool2str( bool flg )
{
return flg ? "Yes" : "No";
}
它在 Visual C++ 和 g++ 中運(yùn)行良好.C++ 標(biāo)準(zhǔn)對(duì)此有何評(píng)論?
It works well in Visual C++ and g++. What does C++ standard say about this?
推薦答案
關(guān)于存儲(chǔ)時(shí)長(zhǎng):
2.13.4普通字符串文字和 UTF-8 字符串文字也稱為窄字符串文字.一個(gè)箭頭字符串文字的類型為n const char 數(shù)組"，其中 n 是如下定義的字符串大小，并且具有靜態(tài)存儲(chǔ)時(shí)長(zhǎng)
2.13.4 Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals. A narrow string literal has type "array of n const char", where n is the size of the string as defined below, and has static storage duration
結(jié)合3.7.1閱讀
3.7.1.
所有沒有動(dòng)態(tài)存儲(chǔ)期，沒有線程存儲(chǔ)期，并且是非本地有靜態(tài)存儲(chǔ)期限.這些物品的儲(chǔ)存應(yīng)持續(xù)到程序 (3.6.2, 3.6.3).
All objects which do not have dynamic storage duration, do not have thread storage duration, and are not local have static storage duration. The storage for these objects shall last for the duration of the program (3.6.2, 3.6.3).
類型:
附件 C
第 2.13.4 條:
更改:字符串文字變?yōu)?const字符串文字的類型從char 數(shù)組"更改為const char 數(shù)組".的類型char16_t 字符串文字從某種整數(shù)類型的數(shù)組"更改為const char16_t 的數(shù)組".這char32_t 字符串文字的類型從某個(gè)整數(shù)類型的數(shù)組"更改為const char32_- 的數(shù)組"噸."寬字符串字面量的類型從wchar_t 數(shù)組"更改為const wchar_t 數(shù)組".
Change: String literals made const The type of a string literal is changed from "array of char " to "array of const char." The type of a char16_t string literal is changed from "array of some-integer-type " to "array of const char16_t." The type of a char32_t string literal is changed from "array of some-integer-type " to "array of const char32_- t." The type of a wide string literal is changed from "array of wchar_t " to "array of const wchar_t."
基本原理:這避免調(diào)用不適當(dāng)?shù)闹剌d函數(shù)，該函數(shù)可能期望能夠修改它的參數(shù).
Rationale: This avoids calling an inappropriate overloaded function, which might expect to be able to modify its argument.
對(duì)原始特征的影響: 改變定義好的特征的語義.轉(zhuǎn)換難度:簡(jiǎn)單的句法轉(zhuǎn)換，因?yàn)樽址淖挚梢赞D(zhuǎn)換為字符*;(4.2).最常見的情況由新的但不推薦使用的標(biāo)準(zhǔn)轉(zhuǎn)換處理:char* p = "abc";//在 C 中有效，在 C++ 中不推薦使用字符* q = expr ?"abc" : "de";//在 C 中有效，在 C++ 中無效
Effect on original feature: Change to semantics of well-defined feature. Difficulty of converting: Simple syntactic transformation, because string literals can be converted to char*; (4.2). The most common cases are handled by a new but deprecated standard conversion: char* p = "abc"; // valid in C, deprecated in C++ char* q = expr ? "abc" : "de"; // valid in C, invalid in C++
使用范圍:有正當(dāng)理由將字符串文字視為潛在指針的程序可修改的內(nèi)存可能很少見.
How widely used: Programs that have a legitimate reason to treat string literals as pointers to potentially modifiable memory are probably rare.
動(dòng)態(tài)分配(標(biāo)準(zhǔn)中AFAIK內(nèi)存區(qū)域的上下文中永遠(yuǎn)不會(huì)使用堆"這個(gè)詞)內(nèi)存需要一個(gè)函數(shù)調(diào)用，該函數(shù)調(diào)用可能早在靜態(tài)內(nèi)存之后的main發(fā)生已分配.
Dynamically allocated (the word 'heap' is never used in context of an area of memory AFAIK in the standard) memory requires a function call that can happen as early as main much after the static memory is allocated.
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