Try and show how toString and StringMaker interact.

Catch_toString.cpp

// ==================== 1) Overload Catch::toString
struct ts_overload { }; // have an overload of toString

namespace Catch {
    std::string toString( const ts_overload& )
    {
        return "ts:ts_overload";
    }
}

TEST_CASE( "toString : ts_overload" )
{
    // Passes. Obviously!
    REQUIRE( Catch::toString( ts_overload() ) == "ts:ts_overload" );
}

TEST_CASE( "toString : vector<ts_overload>" )
{
    typedef std::vector<ts_overload> vec_type;
    // Fails - uses the 'default toString {?}
    REQUIRE( Catch::toString( vec_type(1) ) == "{ ts:ts_overload }" );
}


// ==================== 2) Specialise Catch::StringMaker

struct sm_spec { }; // has a specialisation of StringMaker

namespace Catch {
    template<>
    struct StringMaker<sm_spec> {
        static std::string convert( const sm_spec& )
        {
            return "sm:sm_spec";
        }
    };
};


TEST_CASE( "toString : sm_spec" )
{
    // Invoked the toString<T> templace, which then invokes
    // StringMaker<sm_spec> - so that's fine
    REQUIRE( Catch::toString( sm_spec() ) == "sm:sm_spec" );
}

TEST_CASE( "toString : vector<sm_spec>" )
{
    typedef std::vector<sm_spec> vec_type;
    // vector's StringMaker calls toString, which invokes StringMaker<T>,
    // which does the right thing
    REQUIRE( Catch::toString( vec_type(1) ) == "{ sm:sm_spec }" );
}


// ==================== 3) overload toString AND specialise StringMaker

struct sm_both { };

namespace Catch {
    std::string toString( const sm_both& ) {
        return "ts:sm_both";
    }
    template<>
    struct StringMaker<sm_both> {
        static std::string convert( const sm_both& ) {
            return "sm:sm_both";
        }
    };
}

TEST_CASE( "toString : sm_both" )
{
    // Invoked the toString<T> overload
    REQUIRE( Catch::toString( sm_both() ) == "ts:sm_both" );
}

TEST_CASE( "toString : vector<sm_both>" )
{
    typedef std::vector<sm_both> vec_type;
    // vector's StringMaker calls the template toString - NOT the
    // overload, which invokes StringMaker<sm_both>
    REQUIRE( Catch::toString( vec_type(1) ) == "{ sm:sm_both }" );
}