为什么从字符串常量到'char *'的转换在C中有效但在C++中无效 [英] Why is conversion from string constant to 'char*' valid in C but invalid in C++

问题描述

C++11 标准 (ISO/IEC 14882:2011) 在 § C.1.1 中说:

The C++11 Standard (ISO/IEC 14882:2011) says in § C.1.1:

char* p = "abc"; // valid in C, invalid in C++

对于 C++ 来说，这是可以的，因为指向字符串文字的指针是有害的，因为任何修改它的尝试都会导致崩溃.但为什么它在 C 中有效?

For the C++ it's OK as a pointer to a String Literal is harmful since any attempt to modify it leads to a crash. But why is it valid in C?

C++11 还说:

char* p = (char*)"abc"; // OK: cast added

这意味着如果将强制转换添加到第一个语句中，它将变为有效.

Which means that if a cast is added to the first statement it becomes valid.

为什么强制转换使第二个语句在 C++ 中有效，它与第一个语句有何不同?不还是有害吗?如果是这样，为什么标准说没问题?

Why does the casting makes the second statement valid in C++ and how is it different from the first one? Isn't it still harmful? If it's the case, why did the standard said that it's OK?

推荐答案

在 C++03 之前，您的第一个示例是有效的，但使用了不推荐使用的隐式转换——字符串文字应被视为 char const *，因为你不能修改它的内容(不会导致未定义的行为).

Up through C++03, your first example was valid, but used a deprecated implicit conversion--a string literal should be treated as being of type char const *, since you can't modify its contents (without causing undefined behavior).

从 C++11 开始，已弃用的隐式转换已被正式删除，因此依赖它的代码(如您的第一个示例)不应再编译.

As of C++11, the implicit conversion that had been deprecated was officially removed, so code that depends on it (like your first example) should no longer compile.

您已经注意到允许代码编译的一种方法:虽然隐式转换已被删除，但 显式 转换仍然有效，因此您可以添加强制转换.但是，我不会考虑这种修复"代码.

You've noted one way to allow the code to compile: although the implicit conversion has been removed, an explicit conversion still works, so you can add a cast. I would not, however, consider this "fixing" the code.

真正修复代码需要将指针的类型更改为正确的类型:

Truly fixing the code requires changing the type of the pointer to the correct type:

char const *p = "abc"; // valid and safe in either C or C++.

至于为什么它在 C++ 中被允许(并且仍然在 C 中):仅仅是因为有很多现有的代码依赖于隐式转换，并且破坏该代码(至少没有一些官方警告)显然似乎标准委员会是个坏主意.

As to why it was allowed in C++ (and still is in C): simply because there's a lot of existing code that depends on that implicit conversion, and breaking that code (at least without some official warning) apparently seemed to the standard committees like a bad idea.

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