CODEWITHSUNDEEP
c++templatesshared-ptrmake-shared
jpo38
jpo38

Reputation: 21514

Conditional class used in a make_shared with same parameters

Consider this piece of code creating, based on a condition, a different class instance through a std::make_shared. Note that the two possible classes used (Child1 and Child2) are having the same construction parameters.

class Base
{
public:
    Base( int param )
    {
    }
};

class Child1 : public Base
{
public:
    using Base::Base;
};

class Child2 : public Base
{
public:
    using Base::Base;
};

std::shared_ptr<Base> getPtr( bool cond, int param )
{
    if ( cond )
        return std::make_shared<Child1>(param);
    else
        return std::make_shared<Child2>(param);
}

int main(int argc, char* argv[]) {
    bool permissive = ...;
    Test::getPtr( permissive, argc );
    return 0;
}

Note that permissive is unknown at compilation time.

Is there a way to factorize argument list management in getPtr function with something like:

return std::make_shared<cond?Child1:Child2>(param);

This obviously does not compile but I could not find a way to make something similar work even using templates...

Upvotes: 1

Views: 216

Answers (3)

user7860670
user7860670

Reputation: 37587

Here is generalized solution i came up with while figuring similar problem. Works with several classes and several forwarded arguments. Uses indexing instead of branching. Does not suffer from code duplication.

#include <array>
#include <memory>
#include <utility>
#include <cstddef>

template<typename T, typename... Args> 
::std::shared_ptr<Base> Impl(Args &&... args)
{
    return ::std::make_shared<T>(::std::forward<Args>(args)...);
}

template<typename... T, typename... Args>
::std::shared_ptr<Base> Make(::std::size_t const index, Args &&... args)
{
    return ::std::array{Impl<T, Args...>...}[index](::std::forward<Args>(args)...);
}
...
int main()
{
    auto cond{true};
    auto p_base{Make<Child1, Child2>(cond, 42)};
}

online compiler

Upvotes: 0

Jarod42
Jarod42

Reputation: 217478

With tuple, you might have only one parameter, so, something like:

std::shared_ptr<Base> getPtr(bool cond, /*lot_of_params*/)
{
    std::tuple t{lot_of_params};

    if (cond)
        return std::apply([](auto&&... args){return std::make_shared<Child1>(args...);}, t);
    else
        return std::apply([](auto&&... args){return std::make_shared<Child2>(args...);}, t);
}

Upvotes: 2

jpo38
jpo38

Reputation: 21514

Best solution I could find to avoid duplicating code managing parameters is

class Helper
{
public:
    int param;

    template<class T> 
    std::shared_ptr<Base> create( int param )
    {
        return std::make_shared<T>(param);
    }
};

std::shared_ptr<Base> getPtr( bool cond, int param )
{
    Helper helper{param};
    return cond ? helper.create<Child1>() : helper.create<Child2>();
}

Upvotes: 2

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