const-member-functions

class Image
{
    public:
    int sizeInBytes
    //...
};

Image makeMiniatureImage(Image img) {
    //...
    return img;
}

void removeImageFromDisk(Image img) {
    //...
}

class ImageStorage
{
    std::map<std::string, Image> _imageMap;
    
    Image GetImage(std::string& name) {
        auto itr = _imageMap.find(name);
        if (itr == _imageMap.end())
            throw std::exception();
            
        // developer thought that name is a local variable
        name = name.substr(5);
        // some kind of preview image
        if (name.size() < 2)
            return makeMiniatureImage(itr->second);
        return itr->second;
    }
    
    bool AddImage(std::string& name, Image image)
    {
        auto itr = _imageMap.find(name);
        if (itr == _imageMap.end())
            return false;
        _imageMap[name] = image;
        return true;
    }
    
    size_t CalculateImagesDiskSpace()
    {
        size_t space = 0;
        for (auto& pair : _imageMap)
        {
            space += pair.second.sizeInBytes;
            if (pair.second.sizeInBytes == 0)
            {
                removeImageFromDisk(pair.second);
                _imageMap.erase(_imageMap.find(pair.first));
            }
        }
        return space;
    }
};