Compilation error on passing lambda expression to template function

Question

Why can I not get the template function to accept the lambda expression?

After searching high and low -- I seriously though that this would work, but this C++ code;

template <typename F> int proc(const F& lam)
{
    return lam();
}
void caller()
{
    int i = 42;
    int j = proc( [&i]()->int{ return i/7; } );
}

An I get the following errors;

$ g++ x.cc
x.cc: In function ‘void caller()’:
x.cc:11:44: warning: lambda expressions only available with -std=c++0x or -std=gnu++0x [enabled by default]
x.cc:11:46: error: no matching function for call to ‘proc(caller()::<lambda()>)’
x.cc:11:46: note: candidate is:
x.cc:3:27: note: template<class F> int proc(const F&)

I'm on linux using g++ 4.6.3 and 4.7.2

Does anybody know what I have to do to pass on a lambda expression as a parameter to a receiving template function? -- I don't want to use the std::function -- so my only alternative is to create an ugly functor pattern.

Update: Tried to declare the argument const F& lam, but without success. Update2: added call to compiler...

None -- no flags -- update question with call to compiler... — Soren
– Soren, Commented Sep 27, 2013 at 3:29
You have to use -std=c++11 with g++ 4.7.2 or -std=c++0x with g++ 4.6.3. — Vaughn Cato
– Vaughn Cato, Commented Sep 27, 2013 at 3:32
Thanks -- that does the trick -- now im very confuse what the warning message of "enabled by default" means ..... — Soren
– Soren, Commented Sep 27, 2013 at 3:34

Vaughn Cato · Accepted Answer · 2013-09-27 03:33:43Z

3

Since the lambda isn't an lvalue, you need to pass it by const reference:

template <typename F> int proc(const F& lam)

Make sure to use -std=c++11 with g++ 4.7.2 or -std=c++0x with g++ 4.6.3.

edited Sep 27, 2013 at 3:33

answered Sep 27, 2013 at 3:18

Vaughn Cato

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4 Comments

Soren Over a year ago

Unfortunately that did not solve the problem.. x.cc:11:46: error: no matching function for call to ‘proc(caller()::<lambda()>)’ x.cc:11:46: note: candidate is: x.cc:3:27: note: template<class F> int proc(const F&)

Soren Over a year ago

Thanks @Vaughn -- tried to cut-n-paste your working code -- still have same problem -- suspect it is a compiler version problem -- what version are you using?

Vaughn Cato Over a year ago

@Soren: I'm using g++ 4.7.2

Soren Over a year ago

work with -std=c++11 with g++ 4.7.2 or -std=c++0x with g++ 4.6.3.

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