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std::unique_copy - cppreference.com

template< class InputIt, class OutputIt >
OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );

Copies the elements from the range [first, last), to another range beginning at d_first in such a way that there are no consecutive equal elements. Only the first element of each group of equal elements is copied.

1) Elements are compared using operator==.

3) Elements are compared using the given binary predicate p.

If p does not establish an equivalence relation, the behavior is undefined.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

If *d_first = *first is invalid(until C++20)*first is not writable to d_first(since C++20), the program is ill-formed.

If source and destination ranges overlap, the behavior is undefined.

Given T as the value type of InputIt, if overload (1) or (3) does not satisfy all of the following conditions, the behavior is undefined:

• T is both CopyConstructible and CopyAssignable.
• All following conditions are satisfied:

• OutputIt meets the requirements of LegacyForwardIterator.
• The value type of OutputIt is also T.
• T is CopyAssignable.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to both of them. ​

Output iterator to the element past the last written element.

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) comparisons using operator==.

3,4) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) applications of the predicate p.

For overloads (2,4), there may be a performance cost if the value type of ForwardIt1 is not both CopyConstructible and CopyAssignable.

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

See also the implementations in libstdc++ and libc++.

If InputIt satisfies LegacyForwardIterator, this function rereads the input in order to detect duplicates.

Otherwise, if OutputIt satisfies LegacyForwardIterator, and the value type of InputIt is the same as that of OutputIt, this function compare *d_first to *first.

Otherwise, this function compares *first to a local element copy.

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
    std::string s1 {"A string with mmmany letters!"};
    std::cout << "Before: " << s1 << '\n';
 
    std::string s2;
    std::unique_copy(s1.begin(), s1.end(), std::back_inserter(s2),
                     [](char c1, char c2) { return c1 == 'm' && 'm' == c2; });
 
    std::cout << "After:  " << s2 << '\n';
}

Output:

Before: A string with mmmany letters!
After:  A string with many letters!

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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