Question

The following header works with the commented part as expected when I call the function bat with no arguments:

class Test
{
 public:

  void bat(std::vector<int> k = std::vector<int>()) {}
  //void cat(std::map<int, std::vector<int> > k = std::map<int, std::vector<int> >()) {}

};

But when I try using the cat function in the header:

class Test
{
 public:

  void bat(std::vector<int> k = std::vector<int>()) {}
  void cat(std::map<int, std::vector<int> > k = std::map<int, std::vector<int> >()) {}

};

I get:

test.h:14: error: expected ',' or '...' before '>' token
test.h:14: error: wrong number of template arguments (1, should be 4)
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_map.h:92: error: provided for 'template<class _Key, class _Tp, class _Compare,\
 class _Alloc> class std::map'
test.h:14: error: default argument missing for parameter 2 of 'void Test::cat(std::map<int, std::vector<int, std::allocator<int> >, std::less<int>, std::all\
ocator<std::pair<const int, std::vector<int, std::allocator<int> > > > >, std::vector<int, std::allocator<int> >)'

How come? And are there easy workarounds for this? hopefully not requiring a pointer type change in the interface?

This is my full header:

#ifndef TEST_H
#define TEST_H

#include <map>
#include <vector>
#include <sstream>
#include <iostream>

class Test
{
 public:

  //void bat(std::vector<int> k = std::vector<int>()) {}
  void cat(std::map<int, std::vector<int> > k = std::map<int, std::vector<int> >()) {}

};


#endif

so all the right includes are there. My version of GCC is terribly outdated (well not at home, ill try it at home too) – but at work it’s 4.1.2

Answer 1

The code looks OK, but fails on gcc 4.3.4, see here, but compiles fine with 4.6 onwards (I haven’t tested 4.4 or 4.5). So it looks like the workaround is to use a newer gcc.

#include <map>
#include <vector>

class Test
{
 public:

  void bat(std::vector<int> k = std::vector<int>()) {}
  void cat(std::map<int, std::vector<int> > k = std::map<int, std::vector<int> >
()) {}

};

int main() {

}

Concerning default parameters, it may be an idea to drop them altogether:

class Test {
 public:

  void bat(std::vector<int> k) {}
  void bat() {}
  void cat(std::map<int, std::vector<int> > k) {}
  void cat() {}
};

otherwise, you couple the default parameters to the interface, meaning you cannot change them without requiring re-compilation of all client code.