Why Does `c_str()` Return Garbage When Used Directly on Function Return Strings?

Unveiling the enigma of c_str() in function return strings

The c_str() function is a powerful tool to convert a std::string object into a const char*. However, when applied directly to the return value of a function that returns a string, it can yield mysterious results.

The Essence of the Problem

Consider the following code snippet:

string str = SomeFunction();
const char* strConverted = str.c_str(); // strConverted stores the value of the string properly
const char* charArray= SomeFunction().c_str(); // charArray stores garbage value

static string SomeFunction()
{
    string str;
    // does some string stuff
    return str;
}

In this scenario, strConverted correctly holds the value of the returned string. However, charArray is filled with gibberish. Why does this baffling behavior occur?

Delving into Temporaries

The key lies in the nature of the return value. When SomeFunction() returns a string, the string is actually a temporary object. Temporary objects have a limited lifetime, typically ending at the end of the current block of code.

The Trap of Dangling Pointers

c_str() returns a pointer to the internal buffer of the string. When called on a temporary string directly (e.g., SomeFunction().c_str()), the returned pointer becomes a dangling pointer as the temporary object swiftly vanishes. This dangling pointer is what plagues charArray with unreliable data.

The Solution: Extending Lifetime

To avoid these perils, it's imperative to extend the lifetime of the temporary string. One simple approach is to assign it to a new string variable, as demonstrated in the str_copy example. By doing so, the string object is persisted, ensuring a valid pointer from c_str().

In Conclusion

When dealing with function return strings, caution is advised when using c_str() directly on the return value. To prevent the pitfalls of dangling pointers, assign the return value to a new string variable before invoking c_str(). This guarantees a stable pointer to the string's contents.

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