expression must have a constant value error in c++ [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

Is there a way to initialize an array with non-constant variables? (C++)

I have the following code:

vector<vector<vec2>> vinciP;
    int myLines = -1;
    myLines = drawPolyLineFile("vinci.dat", vinciP);
    if (myLines > -1)
    {
        cout << "\n\nSUCCESS";
        vec2 vPoints[myLines];
        for (int i = 0; i < NumPoints; ++i)
        {
            vPoints[i] = vinciP[0][i];
        }
    }

I'm getting an error on the line 'vec2 vPoints[myLines];' that says expressions must have a constant value. I don't understand why I'm getting this err开发者_如何学编程or, any help?

Is it because myLines could be negative? idk.

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vec2 vPoints[myLines];

Since myLines is not a const expression ((which means, it is not known at compile-time), so the above code declares a variable length array which is not allowed in C++. Only C99 has this feature. Your compiler might have this as an extension (but that is not Standard C++).

The solution to such commom problem is : use std::vector<T> as:

std::vector<vec2> vPoints(myLines);

It should work now.

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Is it because myLines could be negative?
No, It is because myLines is not a compile time constant.

Explanation:

vec2 vPoints[myLines];

Creates an array of variable length, where myLines value will be determined at Run-time. Variable length arrays are not allowed in C++. It was a feature introduced in C99, and C++ Standard does not support it. Some C++ compilers support it as an extension though but it is nevertheless non standard conforming.

For C++ size of an array should be known at compile time and hence must be compile time constant. myLines is not a compile time constant and hence the error.

You should use a std::vector

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vec2 vPoints[myLines];

Array size must be a compile time constant. myLines is not a compile time constant. Instead, allocate the memory using new or even better to use std::vector.

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C++ does not have variable-length arrays. The size of an array must be determined at compile-time. The value of myLines is only known at runtime, so this won't work.

To have arrays whose size is only known at runtime, use std::vector.

std::vector<vec2> vPoints(myLines);

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You are getting that error because static arrays need a static (constant) size. Since the number of components in your vPoints is dynamic, consider using a dynamic array instead. Or better yet stick with vector.