Building Transactional Memory C++ Code in g++

The cppreference website has a (work in progress) page describing transactional memory c++ code. This is the first example on the page

#include <iostream>
#include <vector>
#include <thread>
int f()
{
    static int i = 0;
    synchronized { // begin synchronized block
        std::cout << i << " -> ";
        ++i;       // each call to f() obtains a unique value of i
        std::cout << i << '\n';
        return i; // end synchronized block
    }
}
int main()
{
    std::vector<std::thread> v(10);
    for(auto& t: v)
        t = std::thread([]{ for(int n = 0; n < 10; ++n) f(); });
    for(auto& t: v)
        t.join();
}

Toward the bottom of that page, there is an indication that this builds on gcc (// GCC assembly with the attribute:).

I can't get this to build on g++ 5.3.1:

$ g++ --std=c++11 -fgnu-tm -lpthread trx.cpp 
trx.cpp: In function ‘int f()’:
trx.cpp:7:5: error: ‘synchronized’ was not declared in this scope
     synchronized { // begin synchronized block
     ^

$ g++ --help | grep transaction

$ g++ --version
g++ (Ubuntu 5.3.1-14ubuntu2.1) 5.3.1 20160413

gcc documentation does have a page on transactional memory, but the primitives are different (e.g., the atomic block is __transaction_atomic). The page on cppreference.com conversely appears to be related to N3919, and uses the primitives from there.

How can this code be built with g++?

The transactional_memory link you mention first says:

Compiler support

This technical specification is supported by GCC as of version 6.1 (requires -fgnu-tm to enable).

So you need GCC 6 (and probably also -std=c++1z in addition to -fgnu-tm ....)