C template constructor

Question

I wish to have a non-template class with a template constructor with no arguments   As far as I understand  it s impossible to have it  because it would conflict with the default constructor - am I right    and the workaround is the following   class A     template  lt typename U gt  A U  dummy          Do something           Maybe there is a better alternative for this  or a better workaround

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template lt class    gt struct types using type types    template lt class T gt struct tag using type T    template lt class Tag gt using type t typename Tag  type    the above helpers let you work with types as values   class A     template lt class T gt    A  tag lt T gt          the tag lt T gt  type is a variable with no state besides the type it caries   You can use this to pass a pure-type value into a template function and have the type be deduced by the template function   auto a   A tag lt int gt        You can pass in more than one type   class A     template lt class T  class U  class V gt    A  types lt T U V gt        auto a   A types lt int double std  string gt

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Some points      If you declare any constructor including a templated one   the compiler will refrain from declaring a default constructor    Unless you declare a copy-constructor  for class X one that takes X or X amp  or X const  amp   the compiler will generate the default copy-constructor    If you provide a template constructor for class X which takes T const  amp  or T or T amp  then the compiler will nevertheless generate a default non-templated copy-constructor  even though you may think that it shouldn t because when T   X the declaration matches the copy-constructor declaration  In the latter case you may want to provide a non-templated copy-constructor along with the templated one  They will not conflict  When X is passed the nontemplated will be called  Otherwise the templated   HTH

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Here s a workaround   Make a template subclass B of A  Do the template-argument-independent part of the construction in A s constructor  Do the template-argument-dependent part in B s constructor

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There is no way to explicitly specify the template arguments when calling a constructor template  so they have to be deduced through argument deduction   This is because if you say   Foo lt int gt  f   Foo lt int gt       The  lt int gt  is the template argument list for the type Foo  not for its constructor   There s nowhere for the constructor template s argument list to go   Even with your workaround you still have to pass an argument in order to call that constructor template   It s not at all clear what you are trying to achieve

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You could do this   class C    public      template  lt typename T gt  C T       template  lt typename T gt  T  UseType          static cast lt T  gt  nullptr       Then to create an object of type C using int as the template parameter to the constructor   C obj UseType lt int gt        Since you can t pass template parameters to a constructor  this solution essentially converts the template parameter to a regular parameter  Using the UseType lt T gt    function when calling the constructor makes it clear to someone looking at the code that the purpose of that parameter is to tell the constructor what type to use   One use case for this would be if the constructor creates a derived class object and assigns it to a member variable that is a base class pointer   The constructor needs to know which derived class to use  but the class itself doesn t need to be templated  since the same base class pointer type is always used

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try doing something like  template lt class T  int i gt  class A       A              A this             A  A lt int  1 gt   a               do something           A  A lt float  1 gt   a              do something

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You can create a templated factory function   class Foo   public      template  lt class T gt  static Foo  create      could also return by value  or a smart pointer               return new Foo

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As far as I understand  it s impossible to have it  because it would conflict with the default constructor - am I right     You are wrong  It doesn t conflict in any way  You just can t call it ever

[c++] C++ template constructor

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