std::binder1st, std::binder2nd
Defined in header <functional>
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template< class Fn > class binder1st |
(1) | (deprecated in C++11) (removed in C++17) |
template< class Fn > class binder2nd |
(2) | (deprecated in C++11) (removed in C++17) |
A function object that binds an argument to a binary function.
The value of the parameter is passed to the object at the construction time and stored within the object. Whenever the function object is invoked though operator()
, the stored value is passed as one of the arguments, the other argument is passed as an argument of operator()
. The resulting function object is a unary function.
[edit] Example
#include <cmath> #include <functional> #include <iostream> #include <vector> const double pi = std::acos(-1); // use std::numbers::pi in C++20 int main() { // deprecated in C++11, removed in C++17 auto f1 = std::bind1st(std::multiplies<double>(), pi / 180.0); // C++11 replacement auto f2 = [](double a) { return a * pi / 180.0; }; for (double n : {0, 30, 45, 60, 90, 180}) std::cout << n << "°\t" << std::fixed << "= " << f1(n) << " rad (using binder)\t= " << f2(n) << " rad (using lambda)\n" << std::defaultfloat; }
Output:
0° = 0.000000 rad (using binder) = 0.000000 rad (using lambda) 30° = 0.523599 rad (using binder) = 0.523599 rad (using lambda) 45° = 0.785398 rad (using binder) = 0.785398 rad (using lambda) 60° = 1.047198 rad (using binder) = 1.047198 rad (using lambda) 90° = 1.570796 rad (using binder) = 1.570796 rad (using lambda) 180° = 3.141593 rad (using binder) = 3.141593 rad (using lambda)
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 109 | C++98 | operator() could not modify to argument passed to it
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added overloads to handle this |
[edit] See also
(deprecated in C++11)(removed in C++17) |
binds one argument to a binary function (function template) |