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std::clamp

From cppreference.com
< cpp‎ | algorithm
 
 
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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)

If v compares less than lo, returns lo; otherwise if hi compares less than v, returns hi; otherwise returns v.

1) Uses operator<(until C++20)std::less{}(since C++20) to compare the values.
If T is not LessThanComparable, the behavior is undefined.[1]
2) Uses the comparison function comp to compare the values.

If lo is greater than hi, the behavior is undefined.

  1. If NaN is avoided, T can be a floating-point type.

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.

[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

1) At most two comparisons using operator<(until C++20)std::less{}(since C++20).
2) At most two applications of the comparison function comp.

[edit] Possible implementation

clamp (1)
template<class T>
constexpr const T& clamp(const T& v, const T& lo, const T& hi)
{
    return clamp(v, lo, hi, less{});
}
clamp (2)
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;
}

[edit] 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 Value Std Feature
__cpp_lib_clamp 201603L (C++17) std::clamp

[edit] Example

#include <algorithm>
#include <cstdint>
#include <iomanip>
#include <iostream>
 
int main()
{
    std::cout << " raw   clamped to int8_t   clamped to uint8_t\n";
    for (const int 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

[edit] See also

returns the smaller of the given values
(function template) [edit]
returns the greater of the given values
(function template) [edit]
(C++20)
checks if an integer value is in the range of a given integer type
(function template) [edit]
clamps a value between a pair of boundary values
(niebloid)[edit]