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std::clamp

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
clamp
(C++17)
Permutations
Numeric operations
C library
 
Defined in header <algorithm>
template<class T>
constexpr const T& clamp( const T& v, const T& lo, const T& hi );
(1) (since C++17)
template<class T, class Compare>
constexpr const T& clamp( const T& v, const T& lo, const T& hi, Compare comp );
(2) (since C++17)
1) If v compares less than lo, returns lo; otherwise if hi compares less than v, returns hi; otherwise returns v. Uses operator< to compare the values.
2) Same as (1), but uses comp to compare the values.

The behavior is undefined if the value of lo is greater than hi

Contents

[edit] Parameters

v - the value to clamp
lo,hi - the boundaries to clamp v to
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than the second.

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

 bool cmp(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 T can be implicitly converted to both of them. ​

Type requirements
-
T must meet the requirements of LessThanComparable in order to use overloads (1).

[edit] Return value

Reference to lo if v is less than lo, reference to hi if hi is less than v, otherwise reference to v.

[edit] Complexity

At most two comparisons.

[edit] Possible implementation

First version
template<class T>
constexpr const T& clamp( const T& v, const T& lo, const T& hi )
{
    return clamp( v, lo, hi, std::less<>() );
}
Second version
template<class T, class Compare>
constexpr const T& clamp( const T& v, const T& lo, const T& hi, Compare comp )
{
    return assert( !comp(hi, lo) ),
        comp(v, lo) ? lo : comp(hi, v) ? hi : v;
}

[edit] Notes

Capturing the result of std::clamp by reference if one of the parameters is rvalue produces a dangling reference if that parameter is returned:
int n = -1;
const int& r = std::clamp(n, 0, 255);
// r is dangling

If v compares equivalent to either bound, returns a reference to v, not the bound.

Only works for floating-point T if NaNs are avoided.

[edit] Example

#include <cstdint>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <random>
 
int main()
{
    std::mt19937 g(std::random_device{}());
    std::uniform_int_distribution<> d(-300, 300);
    std::cout << " raw   clamped to int8_t   clamped to uint8_t\n";
    for(int n = 0; n < 5; ++n) {
        int v = d(g);
        std::cout << std::setw(4) << v
                  << std::setw(20) << std::clamp(v, INT8_MIN, INT8_MAX)
                  << std::setw(21) << std::clamp(v, 0, UINT8_MAX) << '\n';
    }
}

Possible output:

.raw   clamped to int8_t   clamped to uint8_t
 168                 127                  168
 128                 127                  128
-137                -128                    0
  40                  40                   40
 -66                 -66                    0


[edit] See also

returns the smaller of the given values
(function template) [edit]
returns the greater of the given values
(function template) [edit]