Scoped temporary values

blocksetting.hh

/**
 *
 * Block-local temporaries
 *
 *  Copyright (C) 2006,2014 Bisqwit (http://iki.fi/bisqwit/)
 * 
 * Example use:
 *    bool in_critical_context = false;
 * 
 *    void example()
 *    {
 *        block_setting<bool> block_marker(in_critical_context, true);
 *        ...
 *        // in_critical_context will be automatically restored to
 *        // whatever it was at function entrance, when the scope is exited.      
 *    }
 *
 * If you wanted to do this with the standard library's tools, you would have
 * to do something like this:
 *
 *    void example2()
 *    {
 *        std::unique_ptr<void,std::function<void(void*)>>
 *            block_marker(nullptr,
 *                         [=](bool& v){bool o=v; v=true;
 *                                      return [=](void*){in_critical_context=o;};
 *                                    }(in_critical_context));
 *        ...
 *    }
 * 
 * Or with hand-written (and always duplicated) code:
 * 
 *   void example3()
 *   {
 *       struct tmp { bool o; ~tmp(){ in_critical_context = o; } } tmp{in_critical_context};
 *       in_critical_context = true;
 *       ...
*    } 
 }
 **/

template<typename T>
class block_setting
{
    T oldvalue;
    T& var;
public:
    block_setting(T& v, const T& tempval) : oldvalue(std::move(v)), var(v) 
    {
        var = tempval;
    }
    ~block_setting()
    {
        var = std::move(oldvalue);
    }
     
    block_setting& operator=(const block_setting&) = delete;
    block_setting(const block_setting&) = delete;
};