std::clamp

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.

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). However, if `NaN` is avoided, `T` can be a floating-point type.

Return value

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

Complexity

At most two comparisons.

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, less{});
}

Second version

template<class T, class Compare>
constexpr const T& clamp( const T& v, const T& lo, const T& hi, Compare comp )
{
    return comp(v, lo) ? lo : comp(hi, v) ? hi : v;
}

Notes

Capturing the result of std::clamp by reference produces a dangling reference if one of the parameters is a temporary and 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.

Feature-test macro:	`__cpp_lib_clamp`

Example

Run this code

#include <cstdint>
#include <algorithm>
#include <iostream>
#include <iomanip>
 
int main()
{
    std::cout << " raw   clamped to int8_t   clamped to uint8_t\n";
    for(int const v: {-129, -128, -1, 0, 42, 127, 128, 255, 256}) {
        std::cout << std::setw(04) << v
                  << std::setw(20) << std::clamp(v, INT8_MIN, INT8_MAX)
                  << std::setw(21) << std::clamp(v, 0, UINT8_MAX) << '\n';
    }
}

Output:

 raw   clamped to int8_t   clamped to uint8_t
-129                -128                    0
-128                -128                    0
  -1                  -1                    0
   0                   0                    0
  42                  42                   42
 127                 127                  127
 128                 127                  128
 255                 127                  255
 256                 127                  255

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

min	returns the smaller of the given values (function template)
max	returns the greater of the given values (function template)
in_range (C++20)	checks if an integer value is in the range of a given integer type (function template)
ranges::clamp (C++20)	clamps a value between a pair of boundary values (niebloid)