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 >

constexpr const T& clamp( const T& v, const T& lo, const T& hi, (2) (since C++17)

Compare comp );

If the value of v is within [lo, hi], returns v; otherwise returns the nearest
boundary.

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.

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.

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¶

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.

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;
}

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

Example¶

// Run this code

#include <algorithm>
#include <cstdint>
#include <iomanip>
#include <iostream>

int main()
{
std::cout << "[raw] "
"[" << INT8_MIN << ',' << INT8_MAX << "] "
"[0," << UINT8_MAX << "]\n";

for (const int v : {-129, -128, -1, 0, 42, 127, 128, 255, 256})
std::cout << std::setw(4) << v
<< std::setw(11) << std::clamp(v, INT8_MIN, INT8_MAX)
<< std::setw(8) << std::clamp(v, 0, UINT8_MAX) << '\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

See also¶

min returns the smaller of the given values
(function template)
max returns the greater of the given values
(function template)
in_range checks if an integer value is in the range of a given integer type
(C++20) (function template)
ranges::clamp clamps a value between a pair of boundary values
(C++20) (niebloid)