std::ranges::clamp

If the value of std::invoke(proj, v) is within [std::invoke(proj, lo), std::invoke(proj, hi)], returns v; otherwise returns the nearest boundary.

The behavior is undefined if std::invoke(proj, lo) is greater than std::invoke(proj, hi).

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

[edit] Parameters

[edit] 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.

[edit] Complexity

At most two comparisons and three applications of the projection.

[edit] Possible implementation

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

[edit] Notes

int n = -1;
const int& r = std::ranges::clamp(n, 0, 255); // r is dangling

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

This function should not be used with both a projection the returns by value and comparator that takes arguments by value unless a move from the projection result type to the comparator parameter type is equivalent to a copy. If the comparison via std::invoke would change the result of projection, the behavior is undefined due to the semantic requirements of std::regular_invocable (subsumed by std::indirect_strict_weak_order).

The standard requires that the value category of the result of the projection be preserved, and proj can only be called on v once, which means that a projection result that is a prvalue has to be cached and moved from twice for the two calls to the comparator.

• libstdc++ does not conform to this and always passes the projection result as an lvalue.
• libc++ used to run the projection twice, which was corrected in Clang 18.
• MSVC STL used to run the projection twice, which was corrected in VS 2022 17.2.

[edit] Example

#include <algorithm>
#include <cstdint>
#include <iomanip>
#include <iostream>
#include <string>
 
using namespace std::literals;
namespace ranges = std::ranges;
 
int main()
{
    std::cout << "[raw] [" << INT8_MIN << ',' << INT8_MAX << "] "
                 "[0" << ',' << UINT8_MAX << "]\n";
    for (int const v : {-129, -128, -1, 0, 42, 127, 128, 255, 256})
        std::cout << std::setw(4) << v
                  << std::setw(11) << ranges::clamp(v, INT8_MIN, INT8_MAX)
                  << std::setw(8) << ranges::clamp(v, 0, UINT8_MAX) << '\n';
    std::cout << std::string(23, '-') << '\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(4) << v
                  << std::setw(11) << ranges::clamp(v, "-128"s, "127"s, {}, stoi)
                  << std::setw(8) << ranges::clamp(v, "0"s, "255"s, {}, stoi)
                  << '\n';
}

[raw] [-128,127] [0,255]
-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
-----------------------
-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