Force a std::vector to free its memory?

Use the swap trick:

#include <vector>

template <typename T>
void FreeAll( T & t ) {
    T tmp;
    t.swap( tmp );
}

int main() {
    std::vector <int> v;
    v.push_back( 1 );
    FreeAll( v );
}

Consider using the swap trick:

std::vector<Shape>().swap(shapes);