CODEWITHSUNDEEP
c++c++11inheritanceconstructorvariadic-templates
Ralph Tandetzky
Ralph Tandetzky

Reputation: 23610

How do I derive and add new arguments to the base version of the constructor?

I'm trying to extend a base class with some data members and hence need some additional constructor arguments in addition to the constructor arguments that my base class needs. I want to forward the first constructor arguments to the base class. Here's what I tried:

#include <string>
#include <utility>

struct X
{
    X( int i_ ) : i(i_) {}
    int i;
};

struct Y : X
{
    template <typename ...Ts>        // note: candidate constructor not viable: 
    Y( Ts&&...args, std::string s_ ) // requires single argument 's_', but 2 arguments 
//  ^                                // were provided
    : X( std::forward<Ts>(args)... )
    , s( std::move(s_) )
    {}

    std::string s;
};

int main()
{
    Y y( 1, "" ); // error: no matching constructor for initialization of 'Y'
//    ^  ~~~~~
}

However, the compiler (clang 3.8, C++14 mode) spits the following error messages at me (the main messages are also in the above source code for reading convenience):

main.cpp:23:7: error: no matching constructor for initialization of 'Y'
    Y y( 1, "" );
      ^  ~~~~~
main.cpp:13:5: note: candidate constructor not viable: requires single argument 's_', but 2 arguments were provided
    Y( Ts&&...args, std::string s_ )
    ^
main.cpp:10:8: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided
struct Y : X
       ^
main.cpp:10:8: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided
1 error generated.

Why is clang trying to tell me, that my templated constructor has only one arguments, even though the number of arguments is variadic? How can I solve this?

Upvotes: 5

Views: 174

Answers (2)

skypjack
skypjack

Reputation: 50550

Here is a possible solution:

#include <string>
#include <utility>
#include<functional>
#include<tuple>
#include<iostream>

struct X
{
    X( int i_ ) : i(i_) {}
    int i;
};

struct Y : X
{
    template<std::size_t... I, typename... A>
    Y(std::integer_sequence<std::size_t, I...>, std::tuple<A...> &&a)
        : X( std::get<I>(a)... ),
          s(std::move(std::get<sizeof...(I)>(a)))
    { }

    template <typename ...Ts>
    Y( Ts&&...args )
        : Y{std::make_index_sequence<sizeof...(Ts)-1>(),
          std::forward_as_tuple(std::forward<Ts>(args)...)}
    { }

    std::string s;
};

int main()
{
    Y y( 1, "foo" );
    std::cout << y.s << std::endl;
}

Upvotes: 3

fnc12
fnc12

Reputation: 2237

You have to move varargs to the end of arguments list.

#include <string>
#include <utility>

struct X
{
    X( int i_ ) : i(i_) {}
    int i;
};

struct Y : X
{
    template <typename ...Ts>        
    Y( std::string s_, Ts&&...args )     //   <==== like this
    : X( std::forward<Ts>(args)... )
    , s( std::move(s_) )
    {}

    std::string s;
};

int main()
{
    Y y( "", 1 ); 
}

Upvotes: 3

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