vector.back() throws “offset out of range” error

Question

void GenerateNativeGamma(vector<float>& gammaNative,
                            float dDetX, 
                            float sdd,
                            float moduleGapX,
                            int nModule,
                            int nChanDetX)
{
    float L = 7*dDetX + (0.78/2+0.11/2);

    gammaNative.push_back(0);

    for (float n = 6.5; n>=-6.5; n--)
    {
        gammaNative.push_back( atanf(L/sdd)-atanf(n*dDetX/sdd));
    }

    gammaNative.push_back(2*atanf(L/sdd));

    if( !gammaNative.empty())
    {
           float lastBonM = gammaNative.back();    // ERROR HAPPENS HERE!!!
           gammaNative.push_back( lastBonM +2*atanf(moduleGapX/2/sdd)  );   
    }

    ......

}

int main()
{
      vector<float>  gammaNative;
      GenerateNativeGamma( gammaNative,  1.02,  869.5, 2.087,47, 24);

}

I am passing a vector by reference into a function from main() ; The purpose is to modify the vector inside it.

Then I have this " offset out of range " error when trying to get the last element via vector.back() like following picture showing:

The vector is not empty, so it should have a back() . Then I have no idea what the problem is here? Or this is not the right way to do it, then how should I modify a vector (dynamic) inside another function? Thanks a lot.

Answer 1

Note that resizing a vector may cause the underlying array to be moved, thus invalidating all pointers, references and iterators to it. When you push_back onto the vector you've passed via reference it can be resized and, of course, the reference will become invalid.

A quick fix would be to use reserve on the vector before calling the function, making sure enough elements can be pushed onto it without reallocation.

A better solution would probably be to return a copy.

EDIT: a quick example

vector<float> vec;
vec.reserve(50);
funcThatAdds50Floats(vec);