c++ - Why Can't I constexpr a bind? -


so want make constexpr functors, though using bind. there i'm missing? why can't bind return constexpr?

given:

struct foo {     int b() const { return _b; }     int a() const { return _a; }     int r() const { return _r; }     const int _b;     const int _a;     const int _r; };

i want to:

constexpr auto sumb = bind(plus<int>(), placeholders::_1, bind(&foo::b, placeholders::_2)); constexpr auto suma = bind(plus<int>(), placeholders::_1, bind(&foo::a, placeholders::_2)); constexpr auto sumr = bind(plus<int>(), placeholders::_1, bind(&foo::r, placeholders::_2));

is there make work?

there no technical obstacle making bind constexpr; example, sprout c++ libraries have constexpr-enabled bind.

however, implementations not permitted add constexpr function signatures not specified in standard, , there has not yet been proposal add constexpr bind aware of (which parts of c++14 standard library , parts made constexpr?). unlikely 1 forthcoming, since bind mostly superseded lambda expressions, of c++17 automatically constexpr:

constexpr auto sumb = [](int x, foo const& y) { return x + y.b(); };

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