Default Call Arguments

Default Call Arguments

Default function call arguments can be specified in function templates just as they are in ordinary functions:

template<typename T> 
void init (T* loc, T const& val = T()) 
{ 
    *loc = val; 
} 

In fact, as this example shows, the default function call argument can depend on a template parameter. Such a dependent default argument is instantiated only if no explicit argument is provided—a principle that makes the following example valid:

class S { 
  public: 
    S(int, int); 
}; 

S s(0, 0); 

int main() 
{ 
   init(&s, S(7, 42));  // T() is invalid for T=S, but the default 
                        // call argument T() needs no instantiation 
                        // because an explicit argument is given 
} 

Even when a default call argument is not dependent, it cannot be used to deduce template arguments. This means that the following is invalid C++:

template<typename T> 
void f (T x = 42) 
{ 
} 

int main() 
{ 
    f<int>(); // OK: T = int 
    f();      // ERROR: cannot deduce T from default call argument 
}