Why is returning `std::string.c_str()` dangerous, and how can I avoid undefined behavior?

Invalidation of std::string.c_str() Return Value

In C , returning a constant character pointer (char*) derived from an instance of std::string using the c_str() method poses a potential issue. This can lead to undefined behavior due to the invalidation of the pointer once the std::string object is destroyed.

Consider the following example:

const char* returnCharPtr() {
    std::string someString;

    // Some processing!

    return someString.c_str();
}

In this code, a std::string object named someString is created locally within the function. After some processing, its internal character array is accessed via the c_str() method and a pointer to this array is returned.

However, when the function scope ends, someString is destroyed and its allocated memory is released. As a result, the pointer returned by c_str() will become a dangling pointer, pointing to memory that is no longer valid.

Resolution

To avoid this issue, there are several possible approaches:

• Return a std::string object: This is the most straightforward solution. By returning the std::string itself, you ensure that a valid copy of the string is created and returned.
• Create a new const char* dynamically: You can allocate memory dynamically to store the string data and return a pointer to that memory. However, remember to deallocate this memory properly when it is no longer needed.
• Customize the returnCharPtr() function: The function can be modified to take an additional argument that is used to store the retrieved string. This eliminates the need to return a const char* directly.

Example

Using the std::string return type:

std::string returnString() {
    std::string someString("something");
    return someString;
}

This function now returns a valid copy of the string, evitando any potential dangling pointers.

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