Namespace & template puzzle

[James W. Walker]

Can someone please explain to me why the following code gives a compile
error when instantiating the sort algorithm?

#include <vector>

namespace
{
struct Foo {
int m;
};
}

static bool operator<( const Foo& inOne, const Foo& inTwo )
{
return inOne.m < inTwo.m;
}

static void test()
{
std::vector<Foo> vec;
std::sort( vec.begin(), vec.end() );
}

It's as if it doesn't understand that I have provided operator< for
Foo. But if I just say

Foo x,y; if (x < y) {... }

instead of sorting, there is no problem. If I move the operator< into
the namespace, then it compiles. So, I know how to fix it, but I'd
like to understand it.

 

[Bob Hairgrove]

Can someone please explain to me why the following code gives a compile
error when instantiating the sort algorithm?

#include <vector>

namespace
{
struct Foo {
int m;
};
}

static bool operator<( const Foo& inOne, const Foo& inTwo )
{
return inOne.m < inTwo.m;
}

static void test()
{
std::vector<Foo> vec;
std::sort( vec.begin(), vec.end() );
}

It's as if it doesn't understand that I have provided operator< for
Foo. But if I just say

Foo x,y; if (x < y) {... }

instead of sorting, there is no problem. If I move the operator< into
the namespace, then it compiles. So, I know how to fix it, but I'd
like to understand it.

By putting Foo() into an anonymous namespace, it is hidden from
everything outside that namespace.

 

[James W. Walker]

By putting Foo() into an anonymous namespace, it is hidden from
everything outside that namespace.

I don't see how that is consistent with what happened. My definition
of operator< for Foo was outside the namespace, and I didn't get a
compile error on that.

 

[Oplec]

I don't see how that is consistent with what happened. My definition
of operator< for Foo was outside the namespace, and I didn't get a
compile error on that.

I'm new to C++, but from what I have read about anonymous namespaces,
whatever is included in an anonymous namespace is only accessable from
within the single translation unit that it is defined in. Maybe that has
something to do with it.

Oplec.

 

[Stephen M. Webb]

Can someone please explain to me why the following code gives a compile
error when instantiating the sort algorithm?

I believe this is an effect of Koenig lookup rules.