std::clamp (3) - Linux Manuals

std::clamp: std::clamp

NAME

std::clamp - std::clamp

Synopsis

Defined in header <algorithm>
template<class T> (1) (since C++17)
constexpr const T& clamp( const T& v, const T& lo, const T& hi );
template<class T, class Compare> (2) (since C++17)
constexpr const T& clamp( const T& v, const T& lo, const T& hi, Compare comp );

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
        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);
comp - 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).

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, 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;
  }

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.

Example

// Run this code

  #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