error: no matching function for call to default copy constructor?

Question

I have a std::map container variable in my class that is populated with objects of my nested-class:

class Logger {
private:
//...
    class Tick{
        ///stores start and end of profiling
        uint32_t start, lastTick,total;
        /// used for total time
        boost::mutex mutexTotalTime;
        ///is the profiling object started profiling?
        bool started;
    public:
        Tick(){
            begin();
        }
        /*
        Tick(const Tick &t){
            start = t.start;
            lastTick = t.lastTick;
            total = t.total;
            started = t.started;
        }
        */
        uint32_t begin();
        uint32_t end();
        uint32_t tick(bool addToTotalTime = false);
        uint32_t addUp(uint32_t value);
        uint32_t getAddUp();

    };
    std::map<const std::string, Tick> profilers_;
//...
public:
//...
Logger::Tick & Logger::getProfiler(const std::string id)
{
    std::map<const std::string, Tick>::iterator it(profilers_.find(id));
    if(it != profilers_.end())
    {
        return it->second;
    }
    else
    {
        profilers_.insert(std::pair<const std::string, Tick>(id, Tick()));
        it = profilers_.find(id);
    }
    return it->second;
}
//...
};

the above code will not compile if I dont provide a copy constructor while I thought the default copy constructor should be already in place?! Am I missing any concept? thanks

That's not a "subclass". It's a nested class. – Lightness Races in Orbit Aug 18 '14 at 08:53 — Lightness Races in Orbit, Aug 18 '14 at 08:53
@buzz: Not if you try to copy `Logger`, it doesn't. – Lightness Races in Orbit Aug 18 '14 at 08:56 — Lightness Races in Orbit, Aug 18 '14 at 08:56

score 3 · Accepted Answer · answered Aug 18 '14 at 08:53

3

The copy constructor may be generated for you only if all members of your class are copyable. In the case of Tick you have an object

boost::mutex mutexTotalTime;

which is not copyable, so the compiler will not generate the copy constructor. Observe that in your commented out copy constructor you do not copy the mutex - because you know you should not. The compiler does not know that.

As a side note, there is no need to explicitly say const for map keys:

std::map<const std::string, Tick> profilers_;

Map keys are always const, and your declaration is completely equivalent to

std::map<std::string, Tick> profilers_;

answered Aug 18 '14 at 08:53

Wojtek Surowka

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The compiler _does_ know that, but it tells you so rather than silently ignoring the mutex. – Lightness Races in Orbit Aug 18 '14 at 08:54
1

Also, [it's not actually equivalent at all](http://stackoverflow.com/a/17639764/560648). – Lightness Races in Orbit Aug 18 '14 at 08:55
@LightnessRacesinOrbit your link says that they are distinct types. Yes they are distinct. Do you have any arguments that they are not equivalent? – Wojtek Surowka Aug 18 '14 at 08:57
If `M` and `M` are distinct types, then writing `M` is not equivalent to writing `M`. – Lightness Races in Orbit Aug 18 '14 at 08:57
1

@LightnessRacesinOrbit What I meant is that for practical purposes when the key is `std::string` there is no difference. Even the link you quoted says "So the difference is largely invisible to you in every way that matters." – Wojtek Surowka Aug 18 '14 at 08:59
@LightnessRacesinOrbit I understand that you define "equivalent" as "not distinct". Weird but let's not discuss semantics :) – Wojtek Surowka Aug 18 '14 at 09:00
There's nothing "weird" about it. There are many ways in which you can observe the distinction, therefore equivalence is lacking. Still, in most ways that matter to the programmer, the two types are interchangeable in a given solution. – Lightness Races in Orbit Aug 18 '14 at 09:02

Angew is no longer proud of SO · Answer 2 · 2014-08-18T08:58:48.580

2

boost::mutex is non-copyable. Since Tick has one as a data member, this makes Tick also non-copyable. This in turn makes the map non-copyable.

So to make Logger copyable, you have to provide your own copy constructor and implement an appropriate duplication of profilers_ in it. Or, perhaps even more appropriately (thanks to @LightnessRacesInOrbit for the suggestion), provide an appropriate copy constructor for Tick instead.

edited Aug 18 '14 at 08:58

answered Aug 18 '14 at 08:52

Angew is no longer proud of SO

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It's more appropriate to define a copy ctor in `Tick` – Lightness Races in Orbit Aug 18 '14 at 08:54

score 2 · Answer 3 · answered Aug 18 '14 at 08:53

The problem is that boost::mutex is not copyable. So if you don't provide a copy constructor, the compiler attempts the generate a default one. That default one needs to copy all of the members, but cannot copy boost::mutex, so it gives up. Your copy constructor does not copy the mutex. Instead it default initializes the new one, so this works.

error: no matching function for call to default copy constructor?

3 Answers3