结构的std :: string成员的EXC_BAD_ACCESS错误 [英] EXC_BAD_ACCESS error for std::string member of a structure

问题描述

在访问类型为std :: string的结构成员时，错误 Bus Error:10 弹出.代码如下.

On accessing a struct member of type std::string, the error Bus Error: 10 popped up. Code is as following.

#include <iostream>
#include <string>

struct KeyValuePair {
    std::string key;
    std::string value;
};

struct KeyValuePair *temp = (struct KeyValuePair *) malloc(sizeof(struct KeyValuePair));


int main(void) {

    temp->value = "|";

    temp->value += "someval|";

    std::cout << temp->value << std::endl;

    return 0;
}

在代码上运行gdb时，在 temp-> value ="|" 行显示以下内容.

Running gdb on the code shows the following at the line temp->value = "|".

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x00007fff8d99e524
0x00007fff898dc7ca in std::string::_M_mutate ()

从以上消息中，我所了解的就是我的代码正在尝试访问无效/未经授权的内存区域.

From above message all I understand is that my code is trying to access an invalid/unauthorized memory area.

我的问题:尽管我已经使用malloc来获取全局变量 temp 的内存区域，但是为什么我无法访问它.我想念的是什么.请帮忙.

My question: Though I have used malloc to obtain a memory area for the global variable temp, why am I unable to access it. What am I missing. Please help.

推荐答案

C和C ++之间有区别:

There is a difference between C and C++:

• 在C语言中， struct 的实例没有固有的不变，它只是位于内存中
• 在C ++中， struct 或 class 的实例具有一组不变量，它们由构造函数建立并由公众维护界面在整个生命周期中

• in C, an instance of a struct has no inherent invariant, it just sits in memory
• in C++, an instance of a struct or class has a set of invariants that are established by the constructor and maintained by the public interface throughout its lifetime

显示在这里:

• malloc 是C构造，只会保留一些原始内存
• new 是C ++构造，不仅会保留一些原始内存，还会调用适当的构造函数，从而建立实例不变式(如果有的话)

• malloc, being a C construct, will just reserve some raw memory
• new, being a C++ construct, will not only reserve some raw memory, but also call the appropriate constructor and thus establish the instance invariants (if any)

注意:如果将 new 与内置类型(例如 int )一起使用，则 new int 实际上不会初始化任何内容.

Note: if using new with built-in types such as int, new int does not actually initialize anything...

因此，在访问temp->value 时，您正在访问未初始化的内存.这是未定义的行为(可能会发生任何事情)，并且在您的情况下，程序会遵循一些野指针并落在禁止访问的内存区域上.

Therefore, when accessing temp->value, you are accessing uninitialized memory. This is Undefined Behavior (anything may happen), and, in your case, the program follows some wild pointer and lands on a memory zone it is forbidden to access.

所以，现在就暂时忘记那些C-ism:常规的C ++构造将确保为您适当地调用构造函数.

So, just forget about those C-isms for now: regular C++ constructs will ensure the constructor is appropriately called for you.

KeyValuePair temp;

int main() {
    // ...
}

或者，如果您确实需要动态分配的值(代表什么?):

Or, if you really need a dynamically allocated value (what for ?):

KeyValuePair* temp = new KeyValuePair();

int main() {
    // ...
}

但是随后您必须考虑在某个时间点在 temp 上调用 delete ，一次，这要复杂得多.没有智能指针，这将是一场失败的游戏.

But then you have to think about calling delete on temp at some point, only once, which is much more complicated. Without a smart pointer, it is a losing game.

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