QPair Class

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

Header: #include <QPair>
qmake: QT += core

Public Types

typedef first_type
typedef second_type

Public Functions

QPair()
QPair(const T1 &value1, const T2 &value2)
QPair(const QPair<TT1, TT2> &p)
QPair(QPair<TT1, TT2> &&p)
void swap(QPair<T1, T2> &other)
QPair<T1, T2> & operator=(const QPair<TT1, TT2> &p)
QPair<T1, T2> & operator=(QPair<TT1, TT2> &&p)

Public Variables

T1 first
T2 second
QPair<T1, T2> qMakePair(const T1 &value1, const T2 &value2)
void swap(QPair<T1, T2> &lhs, QPair<T1, T2> &rhs)
bool operator!=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
bool operator<(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
QDataStream & operator<<(QDataStream &out, const QPair<T1, T2> &pair)
bool operator<=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
bool operator==(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
bool operator>(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
bool operator>=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)
QDataStream & operator>>(QDataStream &in, QPair<T1, T2> &pair)

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.

Member Variable Documentation

T1 QPair::first

The first element in the pair.

T2 QPair::second

The second element in the pair.

Related Non-Members

QPair<T1, T2> qMakePair(const T1 &value1, const T2 &value2)

Returns a QPair<T1, T2> that contains value1 and value2. Example:

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

This is equivalent to QPair<T1, T2>(value1, value2), but usually requires less typing.

void swap(QPair<T1, T2> &lhs, QPair<T1, T2> &rhs)

This is an overloaded function.

Swaps lhs with rhs.

This function was introduced in Qt 5.5.

bool operator!=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)

Returns true if p1 is not equal to p2; otherwise returns false. Two pairs compare as not equal if their first data members are not equal or if their second data members are not equal.

This function requires the T1 and T2 types to have an implementation of operator==().

bool operator<(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)

Returns true if p1 is less than p2; otherwise returns false. The comparison is done on the first members of p1 and p2; if they compare equal, the second members are compared to break the tie.

This function requires the T1 and T2 types to have an implementation of operator<().

QDataStream &operator<<(QDataStream &out, const QPair<T1, T2> &pair)

Writes the pair pair to stream out.

This function requires the T1 and T2 types to implement operator<<().

See also Serializing Qt Data Types.

bool operator<=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)

Returns true if p1 is less than or equal to p2; otherwise returns false. The comparison is done on the first members of p1 and p2; if they compare equal, the second members are compared to break the tie.

This function requires the T1 and T2 types to have an implementation of operator<().

bool operator==(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)

Returns true if p1 is equal to p2; otherwise returns false. Two pairs compare equal if their first data members compare equal and if their second data members compare equal.

This function requires the T1 and T2 types to have an implementation of operator==().

bool operator>(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)

Returns true if p1 is greater than p2; otherwise returns false. The comparison is done on the first members of p1 and p2; if they compare equal, the second members are compared to break the tie.

This function requires the T1 and T2 types to have an implementation of operator<().

bool operator>=(const QPair<T1, T2> &p1, const QPair<T1, T2> &p2)

Returns true if p1 is greater than or equal to p2; otherwise returns false. The comparison is done on the first members of p1 and p2; if they compare equal, the second members are compared to break the tie.

This function requires the T1 and T2 types to have an implementation of operator<().

QDataStream &operator>>(QDataStream &in, QPair<T1, T2> &pair)

Reads a pair from stream in into pair.

This function requires the T1 and T2 types to implement operator>>().

See also Serializing Qt Data Types.

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