C++ templates "expected primary-expression before"

Question

I have code as foolows

template <typename T, typename P>
Nats::MessageCallback registerNatsCallback(P* p)
{
    using std::placeholders::_1;
    using std::placeholders::_2;
    using std::placeholders::_3;
    return std::bind(T, p, _1, _2, _3);
}

I want to use this i this way:

nc.subscribe("Test_1", registerNatsCallback<&App::natsHandler1>(this));

Unfortenly i ended up with compiler error (gcc)

In file included from ..\NATS_CLIENT\main.cpp:2:0:
..\NATS_CLIENT\app.h: In member function 'Nats::MessageCallback
App::registerNatsCallback(P*)':
..\NATS_CLIENT\app.h:28:27: error: expected primary-expression before ',' token
     return std::bind(T, p, _1, _2, _3);

I think that this error has connection with this link, but I didn't see how I can apply 'template' in my situation. I don't have to much experience in templates...

It's never been valid to pass a type as a function argument. — chris, Oct 30 '17 at 20:21

Daniel Trugman · Answer 1 · 2017-10-30T20:37:02.377

You try to pass the function by type only, and it doesn't work. Instead you should pass the method as a pointer, like this:

template <typename T, typename P>
auto registerNatsCallback(T && t, P && p)
{
    using std::placeholders::_1;
    using std::placeholders::_2;
    using std::placeholders::_3;
    return std::bind(std::forward<T>(t), std::forward<P>(p), _1, _2, _3);
}

Notice the use of std::forward() that might improve your implementation.

A full example:

template <typename T, typename P>
auto registerNatsCallback(T && t, P && p)
{
    using std::placeholders::_1;
    using std::placeholders::_2;
    using std::placeholders::_3;
    return std::bind(std::forward<T>(t), std::forward<P>(p), _1, _2, _3);
}

void sample(const char * msg, int a, int b, int c)
{
    std::cout << msg << " " << a << " " << b << " " << c;
}

int main()
{
    auto callback = registerNatsCallback(sample, "hello");
    callback(1,2,3);
}

You forgot the forwarding references for `t` and `p`. – Corristo Oct 30 '17 at 20:34 — Corristo, Oct 30 '17 at 20:34

score 0 · Answer 2 · answered Oct 30 '17 at 20:41

0

Don't use std::bind, it's archaic, inefficient and cumbersome.

Use a lambda instead:

nc.subscribe("Test_1", [this](auto& a, auto& b, auto& c) { return natsHandler1(a, b, c); });

answered Oct 30 '17 at 20:41

rustyx

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C++ templates "expected primary-expression before"

2 Answers2