C++ FAQ / Section 35 / FAQ 35.3

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Revised Jul 4, 2012

[35.3] What's the syntax / semantics for a "function template"?

Consider this function that swaps its two integer arguments:

void swap(int& x, int& y)
{
  int tmp = x;
  x = y;
  y = tmp;
}

If we also had to swap floats, longs, Strings, Sets, and FileSystems, we'd get pretty tired of coding lines that look almost identical except for the type. Mindless repetition is an ideal job for a computer, hence a function template:

template<typename T>
void swap(T& x, T& y)
{
  T tmp = x;
  x = y;
  y = tmp;
}

Every time we used swap() with a given pair of types, the compiler will go to the above definition and will create yet another "template function" as an instantiation of the above. E.g.,

int main()
{
  int         i,j;  /*...*/  swap(i,j);  // Instantiates a swap for int
  float       a,b;  /*...*/  swap(a,b);  // Instantiates a swap for float
  char        c,d;  /*...*/  swap(c,d);  // Instantiates a swap for char
  std::string s,t;  /*...*/  swap(s,t);  // Instantiates a swap for std::string
  ...
}

Note: A "template function" is the instantiation of a "function template".