GCC cannot resolve proper std::accumulate - c++

Question

I'm trying to compile code like this using both GCC 4.4.7 and MSVC 2010:
// test.cpp
// use g++ -c test.cpp to attempt compile this
#include <map>
#include <numeric>
#include <algorithm>
struct A
{
A() : v1(0), v2(0) {}
int v1, v2;
};
typedef std::map<int, A> A_map;
void fill_map(A_map& m); // external function
A process()
{
A_map m;
fill_map(m);
A a;
if(!m.empty())
{
struct Aggregate
{
A operator()(const A& left, const A_map::value_type& right)
{
A result;
result.v1 = left.v1 + right.second.v1;
result.v2 = std::max(left.v2, right.second.v2);
return result;
}
};
A a0;
a = std::accumulate(m.begin(), m.end(), a0, Aggregate());
}
return a;
}
While MSVC2010 compiles this nicely, GCC 4.4.7 gives following error:
test.cpp: In function ‘A process()’:
test.cpp:33: error: no matching function for call to ‘accumulate(std::_Rb_tree_iterator<std::pair<const int, A> >, std::_Rb_tree_iterator<std::pair<const int, A> >, A&, process()::Aggregate)’
Any ideas why so and how to fix this?
Ideas like completely rewrite code using C++11 lambdas do not work - need exactly this code.

Answer 1

C++03 does not allow for the instantiation of templates with local types. If you can't use C++11 or C++14, you can fix that problem by moving the definition of Aggregate outside of the process function. For good measure, make its operator() a const member.
#include <map>
#include <numeric>
#include <algorithm>
struct A
{
A() : v1(0), v2(0) {}
int v1, v2;
};
struct Aggregate
{
A operator()(const A& left, const A_map::value_type& right) const
{
....
}
};
void fill_map(A_map& m); // external function
A process()
{
....
}