CODEWITHSUNDEEP
c++static
Asim
Asim

Reputation: 879

C++: Can you return the value of a local variable from a static method?

If I have something like this:

static const wchar_t* concatenate(const wchar_t* ws1, const wchar_t* ws2) {
    std::wstring s(ws1);
    s += std::wstring(ws2);
    return s.c_str();
}

It wouldn't work because the scope of s is within the static block, so the stack contents will be popped and the memory address to s is no longer valid. So my question is how can I do this?

Upvotes: 1

Views: 3733

Answers (5)

Kricket
Kricket

Reputation: 4179

Replace your return statement with the following:

wchar_t *ret = new wchar_t[s.length()+1];
wcscpy(ret, s.c_str());
return ret;

The function as you wrote it doesn't work, because upon returning, the destructor for the local variable s is called, which frees the memory pointed to by s.c_str().

Upvotes: 2

Filippo Bonato
Filippo Bonato

Reputation: 81

If you want to keep the function signature, try something like:

static const wchar_t* concatenate(const wchar_t* ws1, const wchar_t* ws2) {

    std::wstring s(ws1);
    wchar_t *r;

    s += std::wstring(ws2);

    /*
    * allocate the return value in heap (with room for the null termination)
    */
    r = new wchar_t[s.length() + 1]; 

    /*** copy ***/
    s.copy(r,s.length()+1);

    return r;
}

by the way (as said by others) you may return the entire s object,

Upvotes: 0

Karl Knechtel
Karl Knechtel

Reputation: 61508

The fact that the member function (we don't say "method" in C++) is static doesn't matter. You can return a local variable by value. What you cannot do is return a pointer or a reference to a local variable, or to a temporary value. s.c_str() creates a pointer either to temporary data or to part of the local wstring. So we cannot return that. Returning s (and adjusting the return type to match) is fine, because now we are returning by value, which (conceptually; it may be optimized) makes a copy of the local string on the stack in the return-value "slot".

Upvotes: 1

icecrime
icecrime

Reputation: 76745

The fact that the function is static is irrelevant here. You could return s.c_str() if the s variable was static, however this would be very weird as s would only initialized upon the first call of the function.

My recommendation : just return a std::wstring by value.

std::wstring concatenate(const wchar_t* ws1, const wchar_t* ws2) {
    std::wstring s(ws1);
    s += std::wstring(ws2);
    return s;
}

Upvotes: 5

Dan Breslau
Dan Breslau

Reputation: 11522

Change the function to return std::wstring instead of wchar_t*, and return s.

static std::wstring concatenate(const wchar_t* ws1, const wchar_t* ws2) {
    std::wstring s(ws1);
    s += std::wstring(ws2);
    return s;
}

By the way, this would be equally true for non-static methods.

Upvotes: 12

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