std::ranges::clamp

template< class T, class Proj = std::identity,
          std::indirect_strict_weak_order<std::projected<const T*, Proj>> Comp = ranges::less >
constexpr const T& clamp( const T& v, const T& lo, const T& hi, Comp comp = {}, Proj proj = {} );

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

The behavior is undefined if lo is greater than hi.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists may not be specified when calling any of them.
• None of them is visible to argument-dependent lookup.
• When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

Return value

Reference to lo if the projected value of v is less than the projected value of lo, reference to hi if the projected value of hi is less than the projected value of v, otherwise reference to v.

Complexity

At most two comparisons and three applications of the projection.

Possible implementation

struct clamp_fn {
  template<class T, class Proj = std::identity,
           std::indirect_strict_weak_order<std::projected<const T*, Proj>> Comp = ranges::less>
  constexpr const T& operator()(const T& v, const T& lo, const T& hi,
                                Comp comp = {}, Proj proj = {}) const
  {
      auto&& pv = std::invoke(proj, v);
 
      return std::invoke(comp, std::forward<decltype(pv)>(pv), std::invoke(proj, lo)) ? lo
           : std::invoke(comp, std::invoke(proj, hi), std::forward<decltype(pv)>(pv)) ? hi : v;
  }
};
 
inline constexpr clamp_fn clamp;

Notes

int n = 1;
const int& r = std::ranges::clamp(n-1, n+1);
// r is dangling

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

Example

#include <algorithm>
#include <cstdint>
#include <iostream>
#include <iomanip>
#include <string>
 
using namespace std::literals;
 
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::ranges::clamp(v, INT8_MIN, INT8_MAX)
                  << std::setw(21) << std::ranges::clamp(v, 0, UINT8_MAX) << '\n';
    }
    std::cout << '\n';
 
    // Projection function
    const auto stoi = [](std::string s) { return std::stoi(s); };
 
    // Same as above, but with strings
    for(std::string const v: {"-129", "-128", "-1", "0", "42", "127", "128", "255", "256"}) {
        std::cout << std::setw(04) << v
                  << std::setw(20) << std::ranges::clamp(v, "-128"s, "127"s, {}, stoi)
                  << std::setw(21) << std::ranges::clamp(v, "0"s, "255"s, {}, stoi) << '\n';
    }
}

 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
 
-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

See also