Streaming into default-constructed object

Asperamanca

I want to create a static constant by streaming into a default-constructed object. However, the compiler cannot locate the streaming operator, although I provide it:

@QMap<float, int>& operator<<(QMap<float, int>& map,
const QPair<float, int>& data)
{
(void) map.insert(data.first, data.second);
return map;
}

static const QMap<float, int> s_Map
= QMap<float, int>() << QPair<float,int>(1.0,0);@

For contrast, the following code compiles ("works" would be too strong a word)

@QMap<float, int>& operator<<(QMap<float, int>& map,
const QPair<float, int>& data)
{
(void) map.insert(data.first, data.second);
return map;
}

// Intermediate variable
static QMap<float, int> s_tempMap;

static const QMap<float, int> s_Map
= s_tempMap << QPair<float,int>(1.0,0);@

Why doesn't the first version compile?

I'm using the MinGW provided with Qt 4.8 (4.4.0)

EDIT:
Completely forgot to post the error message:
error: no match for 'operator<<' in 'QMap<float, int>() << QPair<float, int>(((const float&)((const float*)(&1.0e+0f))), ((const int&)((const int*)(&0))))'
candidates are: QMap<float, int>& operator<<(QMap<float, int>&, const QPair<float, int>&)

SGaist

Hi,
You could do something like this

@class FloatIntMap : public QMap<float, int>
{
public:
FloatIntMap() :
QMap<float, int>()
{}

inline FloatIntMap& operator <<(const QPair<float, int>& data)
{
    insert(data.first, data.second);
    return *this;
}

};@

Hope it helps

EDIT
Letting my code but removed my suggestion to avoid mistakes for other forum members.

lgeyer

The temporary <code>QMap<float, int>()</code> is an r-value, whereas <code>QMap<float, int>& operator<<(QMap<float, int>& map, const QPair<float, int>& data)</code> expects an l-value as its first parameter. The solution with the intermedia variable works as <code>static QMap<float, int> s_tempMap</code> is an l-value.

Asperamanca

Isn't QMap<float, int>() just a default-constructed map? I've had this construct work in other cases.

lgeyer

It indeed is, but it is still an r-value ;-)

Each expression in C++ is either an l-value or an "r-value":http://thbecker.net/articles/rvalue_references/section_01.html, regardless of its type; and you cannot bind an r-value to a non-const reference - which you do by passing the temporary to operator<<().

You either add an operator<< taking an r-value reference <code>QMap<float, int>&& map</code> make the first parameter a const reference <code>const QMap<float, int>& map</code> so you can bind an r-value to it, which makes no sense at all, as the map is const then and cannot be modified or you convert your r-value to an l-value by either giving it a name <code>static QMap<float, int> s_tempMap</code> or use an initialization function
@
QMap<float, int> initializeMap()
{
QMap<float, int> map;
map << QPair<float, int>(1.0, 0);

return map;

}

static const QMap<float, int> s_Map = initializeMap();
@

If you want to do it "right" upgrade to Qt5 dev, which has "initializer lists":http://www.stroustrup.com/C++11FAQ.html#init-list for the associative containers like QMap.

Asperamanca

You have led me to an evil idea. How ugly is this:

@
// Regular operator for l-values
QMap<float, int>& operator<<(QMap<float, int>& map,
const QPair<float, int>& data)
{
(void) map.insert(data.first, data.second);
return map;
}

// Operator for r-values. Creates a copy, appends and returns that
QMap<float, int> operator<<(const QMap<float, int>& constMap,
const QPair<float, int>& data)
{
QMap<float, int> map = constMap;
(void) map.insert(data.first, data.second);
return map;
}

// Now this compiles
static const QMap<float, int> s_Map
= QMap<float, int>() << QPair<float,int>(1.0,0);
@

Qt5 is, unfortunately, not an option.

lgeyer

Well, it is as ugly as it more or less strips the const information of your QMap, as people are now allowed to write code like <code>s_Map << QPair<float, int>(2.0, 0)</code> without generating a compiler error (and not modifying the object either).

Asperamanca

I've decided to use your 2nd suggestion, initialization methods.
Thanks!

lgeyer

I think I would prefer initialization methods as well, as they clearly express what you want to do, initialize an object (and thanks to return value optimization they should save a copy constructor invocation as well).

Although Qt 4 does not have initializer list support for its containers GCC 4.4 does, which can be used to provide a generic initalization method which should work with any Qt container (or just add the respective constructors to the containers if modifying Qt itself is an option for you).
@
template <typename Container,
typename Value = typename Container::value_type>
Container initialize(std::initializer_list<Value> values)
{
Container container;
foreach (const Value &value, values)
container.append(value);

return container;

}

template <typename Container,
typename Key = typename Container::key_type,
typename Value = typename Container::mapped_type>
Container initialize(std::initializer_list<QPair<Key, Value>> values)
{
Container container;
foreach (const Value &value, values)
container.insert(value.first, value.second);

return container;

}

const QVector<int> s_Map1 = initialize<QVector<int>>({1, 2, 3});
const QVector<QPair<int, int>> s_Map2 = initialize<QVector<QPair<int, int>>>({{1,1},
{2,2},
{3,3}});
const QMap<float, int> s_Map3 = initialize<QMap<float, int>>({{1.0, 0},
{2.0, 0},
{3.0, 0}});
@

Asperamanca

Cool stuff. Unfortunately, I'm stuck at C++2003 for the time being.