Set default argument(function type) on template function

Question

I have a template function, with several std::function type arguments, I want to default one of these arguments, such that the other programs calling it will not need to specify that input arguments. For example:

template<class T1, class T2>
dataStruct<T1, T2> foo(std::function<T2(T1, T1)> param_1, 
                       std::function<T2(T1, T1)> param_2,
                       std::function<double(T1, T1)> param_3) {...}

I want to set param_3 to have a default value of 1.0, so if I want to call this template function, I only need call it like this:

dataStruct<int, int> solution = foo(input_fun1, input_fun2);

such that I don't need to specify param_3 if its value is just default 1.0. I know I can overload it with less parameters, but any way to skip this overload? like set default value in this template function directly? std::function<double(T1, T1)> param_3 = setToOne() something like this?

You could set the default value of param_3 to a static function that will take your parameters and always return 1.0, but depending on your implementation, it would probably end up being less readable than the answer below. — user2209008
– user2209008, Commented Jul 28, 2015 at 22:22

Miles Budnek · Accepted Answer · 2015-07-28 23:53:13Z

3

You could use a lambda as a default parameter:

template<class T1, class T2>
dataStruct<T1, T2> foo(std::function<T2(T1, T1)> param_1,
                       std::function<T2(T1, T1)> param_2,
                       std::function<double(T1, T1)> param_3 = [](T1, T1) { return 1.0; })
{
    //...
}

edited Jul 28, 2015 at 23:53

answered Jul 28, 2015 at 23:13

Miles Budnek

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4 Comments

Rusty Shackleford Over a year ago

Extrapolated to the OP's code:

template<class T1, class T2> dataStruct<T1, T2> foo(     std::function<T2(T1, T1)> param_1,     std::function<T2(T1, T1)> param_2,     std::function<double(T1, T1)> param_3 = [](T1, T1) { return 1.0; }) { ... }

Miles Budnek Over a year ago

You're right, I should have done that in the first place. Edited into the answer.

Emerson Xu Over a year ago

Using lambda, I still need to specify this param_3 when call foo(), right? I tried on my complier and return no match function error.

Rusty Shackleford Over a year ago

@EmersonXu: Your param_N may be a function pointer (e.g. T2 (*)(T1, T1) and not a std::function<T2(T1, T1)>, so you need to help the compiler figure out which instantiation of foo you want to call. One way to do this is to explicitly specify the template arguments T1 and T2 when calling foo: e.g. foo<int, std::string>(p1, p2). Another way is to convert your function pointers to std::function before calling foo: e.g. std::function<int, std::string> f1 = p1;, and then pass f1 et al. to foo instead of p1 et al.

R Sahu · Accepted Answer · 2015-07-29 22:35:01Z

1

I want to set param_3 to have a default value of 1.0.

I don't think you can do that. The value of a std::function<double(T1, T1)> cannot be 1.0. One option is to overload the function.

template <class T1, class T2>
dataStruct<T1, T2> foo(std::function<T2(T1, T1)> param_1, 
                       std::function<T2(T1, T1)> param_2,
                       double param_3 = 1.0) { ... }

edited Jul 29, 2015 at 22:35

answered Jul 28, 2015 at 22:18

R Sahu

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1 Comment

MSalters Over a year ago

There are plenty of ways you can achieve this, e.g. this somewhat silly idea or as the other answer points out, a lambda.

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