Compiler error C3493: 'func' cannot be implicitly captured because no default capture mode has been specified

Question

Can you help me resolve this compiler error?

template<class T>
static void ComputeGenericDropCount(function<void(Npc *, int)> func)
{
    T::ForEach([](T *what) {
        Npc *npc = Npc::Find(what->sourceId);

        if(npc)
            func(npc, what->itemCount); // <<<<<<< ERROR HERE
            // Error    1   error C3493: 'func' cannot be implicitly captured because no default capture mode has been specified

    });
}

static void PreComputeNStar()
{
     // ...
    ComputeGenericDropCount<DropSkinningNpcCount>([](Npc *npc, int i) { npc->nSkinned += i; });
    ComputeGenericDropCount<DropHerbGatheringNpcCount>([](Npc *npc, int i) { npc->nGathered += i; });
    ComputeGenericDropCount<DropMiningNpcCount>([](Npc *npc, int i) { npc->nMined += i; });
}

I can't understand why it's giving me the error and I don't know how to fix it. ComputeGenericDropCount(auto func) doesn't work either.

Solution

You need to specify how to capture func into the lambda.

[] don't capture anything

[&] capture-by-reference

[=] capture-by-value (copy)

T::ForEach([&](T *what) {

I'd also recommend that you should send func by const reference.

static void ComputeGenericDropCount(const function<void(Npc *, int)>& func)

Comments

[Andreas Bonini]

This doesn't work and gives me the same error; also I thought that [] means "capture nothing"; [&] means "capture all upvalues by reference, [*] means "capture all upvalues by copy", and so on.. But I think this only affects how upvalues are captured, ie, it will only affect code inside the lambda

[ronag]

You need to capture func in-order to use it inside the lambda, which you do. I don't see how you could get the same error. I'll try it myself once i get home.

[Andreas Bonini]

Sorry! I added the & to the wrong lambdas, the callbacks from ComputeGenericDropCount, instead of ForEach, as you said. Now everything makes sense: it needed to know how to capture func, of course!! Thanks again.

[DawnSong]

& is what I need. Thanks