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Showing content from https://cplusplus.com/reference/map/map/operator=/ below:

public member function

<map>

std::map::operator= copy (1)

 map& operator= (const map& x);

copy (1)

map& operator= (const map& x);

move (2)

map& operator= (map&& x);

initializer list (3)

map& operator= (initializer_list<value_type> il);

Copy container content

Assigns new contents to the container, replacing its current content.

Copies all the elements from

into the container, changing its

size

accordingly.

The container preserves its current allocator, which is used to allocate additional storage if needed.

The

copy assignment

(1) copies all the elements from

into the container (with

preserving its contents).

The move assignment (2) moves the elements of x into the container (x is left in an unspecified but valid state).

The initializer list assignment (3) copies the elements of il into the container.

The new container size is the same as the size of x (or il) before the call.

The container preserves its current allocator, except if the allocator traits indicate x's allocator should propagate. This allocator is used (through its traits) to allocate or deallocate if there are changes in storage requirements, and to construct or destroy elements, if needed.

The elements stored in the container before the call are either assigned to or destroyed.

Parameters

x: A map object of the same type (i.e., with the same template parameters, key, T, Compare and Alloc).
il: An initializer_list object. The compiler will automatically construct such objects from initializer list declarators.
Member type value_type is the type of the elements in the container, defined in map as an alias of pair<const key_type, mapped_type> (see map member types).

Return value*this
Example

// assignment operator with maps
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> first;
  std::map<char,int> second;

  first['x']=8;
  first['y']=16;
  first['z']=32;

  second=first;                // second now contains 3 ints
  first=std::map<char,int>();  // and first is now empty

  std::cout << "Size of first: " << first.size() << '\n';
  std::cout << "Size of second: " << second.size() << '\n';
  return 0;
}

Output:

Size of first: 0
Size of second: 3

Complexity For the copy assignment (1): Linear in sizes (destructions, copies).
For the move assignment (2): Linear in current container size (destructions).*
For the initializer list assignment (3): Up to logarithmic in sizes (destructions, move-assignments) -- linear if il is already sorted.
* Additional complexity for assignments if allocators do not propagate.

Iterator validity All iterators, references and pointers related to this container are invalidated.

In the move assignment, iterators, pointers and references referring to elements in x are also invalidated.

Data races All copied elements are accessed.
The move assignment (2) modifies x.
The container and all its elements are modified.

Exception safetyBasic guarantee: if an exception is thrown, the container is in a valid state.
If allocator_traits::construct is not supported with the appropriate arguments for the element constructions, or if value_type is not copy assignable (or move assignable for (2)), it causes undefined behavior.

See also

map::insert: Insert elements (public member function)

map::operator[]: Access element (public member function)

map::map: Construct map (public member function)

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