C++ template compilation error: expected primary-expression before ‘>’ token

Question

This code compiles and works as expected (it throws at runtime, but never mind):

#include <iostream>
#include <boost/property_tree/ptree.hpp>

void foo(boost::property_tree::ptree &pt) 
{
    std::cout << pt.get<std::string>("path"); // <---
}

int main()
{
    boost::property_tree::ptree pt;
    foo(pt);
    return 0;
}

But as soon as I add templates and change the foo prototype into

template<class ptree>
void foo(ptree &pt)

I get an error in GCC:

test_ptree.cpp: In function ‘void foo(ptree&)’:
test_ptree.cpp:7: error: expected primary-expression before ‘>’ token

but no errors with MSVC++! The error is in the marked line <---. And again, if I change the problem line into

--- std::cout << pt.get<std::string>("path"); // <---
+++ std::cout << pt.get("path", "default value");

the error disappears (the problem is in explicit <std::string>).

Boost.PropertyTree requires Boost >= 1.41. Please help me to understand and fix this error.

---

See Templates: template function not playing well with class’s template member function — a similar popular question containing other good answers and explanations.

Solution

You need to do:

std::cout << pt.template get<std::string>("path");

Use template in the same situation as typename, except for template members instead of types.

(That is, since pt::get is a template member dependent on a template parameter, you need to tell the compiler it's a template.)

Comments

[Chance]

Nice. Do you have a specific resource you use for template usage? I ask out of curiosity. None of my books seemed to address this issue.

[GManNickG]

@Chance: I don't, sadly, though you might find one off this list, namely C++ Templates, The Complete Guide.

[HelloGoodbye]

Why do you need to tell the compiler it's a template? Shouldn't the compiler be able to figure that out by itself?

[GManNickG]

@HelloGoodbye: I don't remember if it's theoretically possible in C++ for the compiler to always deduce it correctly, probably not. The problem is that without template, it could be parsed as (pt.get) < std::string > ("path");, that is comparisons. In this case, comparisons for type names isn't possible so it could figure out, but in general how would you parse pt.val < 5 > (10)? Adding template tells the compiler directly "this is a function call".

[HelloGoodbye]

Ah, okay, so it is the old incompatibility problem with comparison operators and template notation again... :P Well, that makes sense. I guess the D programming language has the upper hand here with its (...) template syntax instead of C++' <...> syntax :)