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std::ranges::clamp(3) C++ Standard Libary std::ranges::clamp(3) NAME std::ranges::clamp - std::ranges::clamp Synopsis Defined in header <algorithm> Call signature template< class T, class Proj = std::identity, std::indirect_strict_weak_order<std::projected<const T*, Proj>> Comp = (since ranges::less > C++20) constexpr const T& clamp( const T& v, const T& lo, const T& hi, Comp comp = {}, Proj proj = {} ); If the value of v is within [lo, hi], returns v; otherwise returns the nearest boundary. 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 cannot be specified when call- ing 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 in- hibited. In practice, they may be implemented as function objects, or with special compiler extensions. Parameters v - the value to clamp lo, hi - the boundaries to clamp v to comp - the comparison to apply to the projected elements proj - the projection to apply to v, lo and hi Return value Reference to lo if the projected value of v is less than the pro- jected value of lo, reference to hi if the projected value of hi is less than the pro- jected 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 = 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::for- ward<decltype(pv)>(pv))) return hi; return v; } }; inline constexpr clamp_fn clamp; Notes Capturing the result of std::ranges::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::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 pro- jection 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 projec- tion 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. Example // Run this code #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'; } Output: [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 See also ranges::min returns the smaller of the given values (C++20) (niebloid) ranges::max returns the greater of the given values (C++20) (niebloid) in_range checks if an integer value is in the range of a given integer type (C++20) (function template) clamp clamps a value between a pair of boundary values (C++17) (function template) http://cppreference.com 2024.06.10 std::ranges::clamp(3)
NAME | Synopsis | Parameters | Return value | Complexity | Possible implementation | Notes | lvalue. | Example | Output: | See also
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