将 std::to_string(x).c_str() 存储在成员变量中会产生垃圾 [英] Storing std::to_string(x).c_str() in member variable produces garbage

问题描述

我有一个非常简单的 C++ 程序:

I have this very simple C++ program:

using namespace std;

class TheClass
{
private:
    const char *_numberString;

public:
    TheClass(int number)
    {
        _numberString = to_string(number).c_str();
    }

    operator const char *()
    {
        return _numberString;
    }
};

int main(int argc, const char * argv[])
{
   TheClass instance = 123;
   cout << (const char *)instance << endl;

   return 0;
}

当我在 Xcode 中运行它时，它会记录 \367\277_\377.但是，如果我将其更改为:

When I run it in Xcode, it logs \367\277_\377. If I change it to this however:

using namespace std;

class TheClass
{
public:   // Change 1/2
    const char *_numberString;

public:
    TheClass(int number)
    {
        _numberString = to_string(number).c_str();
    }

    operator const char *()
    {
        return _numberString;
    }
};

int main(int argc, const char * argv[])
{
    TheClass instance = 123;
    instance._numberString = to_string(123).c_str();   // Change 2/2
    cout << (const char *)instance << endl;

    return 0;
}

它像它应该的那样记录123.我看不出我做错了什么.即使我将 123 更改为另一个数字，也会记录完全相同的内容.

it logs 123 like it should. I can't see what I'm doing wrong. Even if I change 123 to another number the exact same thing is logged.

推荐答案

此时

 _numberString = to_string(number).c_str();

您正在存储一个指向临时 std::string 值的内部数据的指针，该值在该行代码之后无效.

you are storing a pointer to the interned data of a temporary std::string value, that is invalidated after that line of code.

访问 _numberString 有效调用未定义的行为.

Accessing _numberString effectively calls undefined behavior.

正如评论中提到的，将 _numberString¹ 成员保留为 const char* 是没有意义的.改用 std::string 成员:

As mentioned in comments, there's no point to keep the _numberString¹ member as const char*. Use a std::string member instead:

class TheClass {
private:
     std::string  numberString_;

public:
    TheClass(int number) : numberString_(to_string(number)) {
    }

    operator const std::string& () {
         return numberString_;
    }
};

<小时>

_{1) 你不应该使用前缀 _ 作为类成员名称，这是为编译器和标准实现内部函数保留的.如果你不喜欢像 m_ 这样的模式或其他前缀约定(像我一样)，只需使用后缀 _ 如我的示例所示.}

---

_{1) You shouldn't use prefixed _ for class member names, that's reserved for compiler and standard implementation intrinsics. If you dislike patterns like m_ or other prefix conventions (like me), just use a postfix _ as shown in my sample.}

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