Q:

std::function parameter is incomplete type not allowed

I am trying to assign a lambda to an std::function, like so:
std::function<void>(thrust::device_vector<float>&) f;
f = [](thrust::device_vector<float> & veh)->void
{   
    thrust::transform( veh.begin(), veh.end(), veh.begin(), tanh_f() );
};

And I get an error: 
 error: incomplete type is not allowed
 error: type name is not allowed

I think it refers to thrust::device_vector<float>.  I tried typenaming and typedefining the parameter:
typedef typename thrust::device_vector<float> vector;
std::function<void>(vector&) f;
f = [](vector & veh)->void
{   
    thrust::transform( veh.begin(), veh.end(), veh.begin(), tanh_f() );
};

To no avail.  However, if I just use the lambda (without the std::function) it works:
typedef typename thrust::device_vector<float> vector;
auto f = [](vector & veh)->void
{   
    thrust::transform( veh.begin(), veh.end(), veh.begin(), tanh_f() );
};

What am I missing? 
PS: I am compiling using nvcc release 6.5, V6.5.12 and g++ (Debian 4.8.4-1) 4.8.4

A:

You are using the wrong syntax. 
Try std::function<void(thrust::device_vector<float>&)> f; instead
std::function<void(thrust::device_vector<float>&)> f; declares a variable with type std::function<void(thrust::device_vector<float>&)>, which is a function object that takes thrust::device_vector<float>& and return void
g++ give you the incomplete type error because std::function<void> is not a valid template instantiation. 
clang++ give you a better error message telling you that std::function<void>() f; is an invalid variable declaration:
main.cpp:11:28: error: expected '(' for function-style cast or type construction
    std::function<void>(int) f;
                        ~~~^
1 error generated.