为什么从字符串转换常数为'字符*'有效的C，但无效在C ++中 [英] Why is conversion from string constant to 'char*' valid in C but invalid in C++

问题描述

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

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 ++没关系作为一个指向字符串文字是有害的，因为任何试图修改它会导致崩溃。但是为什么它有效使用C？

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?

在C ++ 11也说：

The C++11 says also:

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

这意味着如果一个铸件添加到第一语句它变为有效。

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

为什么铸造使得第二条语句在C ++中有效，它是如何从第一个有什么不同？是不是还是有害？如果是的话，为什么没有标准说，这是确定？

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

向上穿过C ++ 03，你的第一个例子是有效的，但使用了德precated隐式转换 - 一个字符串文字应被视为是类型字符常量* ，因为你不能修改其内容（而不会导致不确定的行为）。

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).

由于C ++ 11，依赖于它（如您的第一个例子）的隐式转换已被德precated被正式删除，因此code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.

您已经注意到的一种方式，让code编译：虽然隐式转换已被删除，一个的明确的转换仍然有效，所以你可以添加一个演员。我的不的，但是，考虑这个固定的code。

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.

真正固定code需要改变指针的类型正确的类型：

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++.

至于为什么它被允许在C ++（现在仍然是在C）：只是因为有很多现有的code的是依赖于隐式转换，并打破了code（至少没有一些官员警告）显然似乎标准委员会像一个坏主意。

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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