QPair Class

The QPair class is a template class that stores a pair of items. More...

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Public Types

Public Functions

Public Variables

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Detailed Description

The QPair class is a template class that stores a pair of items.

QPair<T1, T2> can be used in your application if the STL pair type is not available. It stores one value of type T1 and one value of type T2. It can be used as a return value for a function that needs to return two values, or as the value type of a generic container.

Here's an example of a QPair that stores one QString and one double value:

QPair<QString, double> pair;

The components are accessible as public data members called first and second. For example:

pair.first = "pi";
pair.second = M_PI;

Note, however, that it is almost always preferable to define a small struct to hold the result of a function with multiple return values. A struct trivially generalizes to more than two values, and allows more descriptive member names than first and second:

struct Variable {
    QString name;
    double value;
};
Variable v;
v.name = "pi";
v.value = M_PI;

The advent of C++11 automatic variable type deduction (auto) shifts the emphasis from the type name to the name of functions and members. Thus, QPair, like std::pair and std::tuple, is mostly useful in generic (template) code, where defining a dedicated type is not possible.

QPair's template data types (T1 and T2) must be assignable data types. You cannot, for example, store a QWidget as a value; instead, store a QWidget *. A few functions have additional requirements; these requirements are documented on a per-function basis.

See also Container Classes.

Member Type Documentation

typedef QPair::first_type

The type of the first element in the pair (T1).

See also first.

typedef QPair::second_type

The type of the second element in the pair (T2).

See also second.

Member Function Documentation

QPair::QPair()

Constructs an empty pair. The first and second elements are initialized with default-constructed values.

QPair::QPair(const T1 &value1, const T2 &value2)

Constructs a pair and initializes the first element with value1 and the second element with value2.

See also qMakePair().

QPair::QPair(const QPair<TT1, TT2> &p)

Constructs a pair from the other pair p, of types TT1 and TT2. This constructor will fail if first cannot be initialized from p.first or if second cannot be initialized from p.second.

This function was introduced in Qt 5.2.

See also qMakePair().

QPair::QPair(QPair<TT1, TT2> &&p)

Move-constructs a QPair instance, making it point to the same object that p was pointing to.

This function was introduced in Qt 5.2.

void QPair::swap(QPair<T1, T2> &other)

Swaps this pair with other.

Equivalent to

qSwap(this->first, other.first);
qSwap(this->second, other.second);

Swap overloads are found in namespace std as well as via argument-dependent lookup (ADL) in the namespace of T .

This function was introduced in Qt 5.5.

QPair<T1, T2> &QPair::operator=(const QPair<TT1, TT2> &p)

Copies pair p into this pair.

This function was introduced in Qt 5.2.

See also qMakePair().

QPair<T1, T2> &QPair::operator=(QPair<TT1, TT2> &&p)

Move-assigns pair p into this pair instance.

This function was introduced in Qt 5.2.

QList<QPair<int, double> > list;
list.append(qMakePair(66, M_PI));