Alright, I am really confused.

// At function scope
    // This is a stringstream
    scoress << "Score:\n" << player->GetScore();
    this->printString( scorerect.x + 4,
                       scorerect.y + 4,
                       SDL_GetVideoSurface(),
                       scoress.str(),
                       color[whitea],
                       color[blackb] );
    scoress.str("");

// ...

// Outside function scope
void Graphics::printString( int x, int y, SDL_Surface *dest, std::string sz, SDL_Color fgcolor, SDL_Color bgcolor )
{
    SDL_Rect offset;
    offset.x = x;
    offset.y = y;

    int width = 0, height = 10;
    int lineskip = TTF_FontLineSkip( font );
    std::vector<std::string> vlines;

    int n = 0;
    std::string szsub;
    // Break up the lines
    while( n != -1 ) {
        // Find a newline
        n = sz.find( '\n', 0 );
        // Split up at the newline
        szsub = sz.substr( 0, n );
        if( n != -1 ) sz = sz.substr( n+1, 1 );
        // Add the line to the vector
        vlines.push_back( szsub );

        // Get the size of the rendered text
        int w = 0;
        TTF_SizeText( font, szsub.c_str(), &w, &height );
        if( w > width ) width = w;
    }

    height = (vlines.size()-1) * lineskip + height;

    // Draw
    SDL_Surface *temp = NULL;
    SDL_Surface *text = NULL;

    text = SDL_CreateRGBSurface( SDL_SWSURFACE, width, height,
                                 dest->format->BitsPerPixel,
                                 dest->format->Rmask,
                                 dest->format->Gmask,
                                 dest->format->Bmask,
                                 dest->format->Amask );

    offset.w = width;
    offset.h = height;
    SDL_FillRect( text, NULL, SDL_MapColor( SDL_GetVideoSurface()->format, bgcolor ) );

    SDL_Rect rect;
    for( int i = 0; i < vlines.size(); i++ ) {
        temp = TTF_RenderText_Blended( font, vlines.c_str(), fgcolor );
        rect.x = 0;
        rect.y = i * lineskip;

        SDL_BlitSurface( temp, NULL, text, &rect );
    }

    SDL_BlitSurface( text, NULL, dest, &offset );
}

player->GetScore(); returns an integer. By debugging into stdout.txt, I managed to find out that the players score is updated correctly. The game however displays the wrong score (roughly the score divided by 10), and sometimes resets the score randomly to 1. I have no idea how this is possible. Can someone please help me?

Did you try creating a new stringstream each time to format your score rather than reusing an existing one?

I found my error :) Thanks for your reply anyway though.

I mistyped something from this gamedev tutorial.

if( n != -1 ) sz = sz.substr( n+1, 1 );

should be according to the tutorial

if( n != -1 ) sz = sz.substr( n+1, -1 );

but that gives me a compiler warning. This line works though:

if( n != -1 ) sz = sz.substr( n+1 );