Return my struct as an array in C++, OPENGL

Question

I have a problem with my function. I can't seem to make it return an array of a struct.

Here is the MyApp.h header file:

    struct Vertex
{
    glm::vec3 p;
    glm::vec3 c;
};

class CMyApp
{

public:
    CMyApp(void);
    ~CMyApp(void);

    Vertex[] DrawCircle(int cx, int cy);
...

It underlines the DrawCircle and "expects a ';'".

Here is the MyApp.cpp (Of course, header included):

Vertex[] CMyApp::DrawCircle(int cx, int cy) {
    Vertex result[42];

    result[0] = { glm::vec3((float)cx, (float)cy, 0.0), glm::normalize(glm::vec3(0, 0, 1)) };

    for (int ii = 1; ii < 21; ii++) {
        float theta = 2.0f * 3.1415926f * float(ii) / float(20);
        float x = 0.5 * cosf(theta);
        float y = 0.5 * sinf(theta);
        result[ii].p = glm::vec3(x, y, 0.0);
        result[ii].c = glm::normalize(result[ii].p);
    }

    result[21] = { glm::vec3((float)cx, (float)cy, 2.0), glm::normalize(glm::vec3(0, 0, 1.0)) };

    for (int ii = 22; ii < 42; ii++) {
        float theta = 2.0f * 3.1415926f * float(ii) / float(20);
        float x = 0.5 * cosf(theta);
        float y = 0.5 * sinf(theta);
        result[ii].p = glm::vec3(x, y, 2.0);
        result[ii].c = glm::normalize(result[ii].p);
    }

    return result;
}

Same underline here under the function name's DrawCircle for expected ";".

If I remove the array marks then the only error is the return statement. I want to return as an array tho.

Thanks for help in advance.

Use pointer and feedback the length by reference in parameter vertex* function(int& length), or use the std::vector which had been described in answer. — Tokenyet
– Tokenyet, Commented Nov 19, 2016 at 11:05

Community · Accepted Answer · 2020-06-20 09:12:55Z

You cannot return a local array. Such an array is allocated on the stack; when the function returns, all its content is available for other stack variables. If you use it after the call, its content is likely to be corrupted.

So

Vertex[] CMyApp::DrawCircle(int cx, int cy) {
    Vertex result[42];
    return result;
}

is undefined behavior for the compiler.

You should use a vector instead. Its move constructor makes it efficient for returning many results organized as an array.

std::vector<Vertex> CMyApp::DrawCircle(int cx, int cy) {
    std::vector<Vertex> result;
    result.reserve(42);
    // same content than your original code.
    ...
    return result;
}

Note that if you declare

class CMyApp
{

public:
    CMyApp(void);
    ~CMyApp(void);

    typedef Vertex ArrayOfVertices[];
    ArrayOfVertices DrawCircle(int cx, int cy);
};

You obtain the error message:

error: ‘DrawCircle’ declared as function returning an array

ArrayOfVertices DrawCircle(int cx, int cy);

datenwolf · Accepted Answer · 2016-11-19 10:29:09Z

0

I want to return as an array tho.

You can't. C and C++ don't allow it. What you can do in C++ is returning a std::vector which you should use instead of plain arrays anyway.

answered Nov 19, 2016 at 10:29

datenwolf

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