问题
Some of the standard iomanip
functions take take a parameter.
I'd like to know how this is accomplished, for instance, can I do something similar with a function? That's really the solution that I needed for this answer, but I couldn't figure out how to do this.
When I looked up the definition for setw
function for example in http://en.cppreference.com it lists the return type as "unspecified", and it also only lists one argument, rather than also taking a stream&
parameter. How does this work?
Note to the reader:
This question has an excellent answer but to the individual looking up this question; this answer will only be useful in combination with the following functionality provided by ios_base
:
- xalloc
- iword
- pword
回答1:
Here is a simple example of a user-defined manipulator that takes one parameter defined using a class:
#include <iostream>
class putX // injects some `X`s into the stream
{
std::size_t _n;
public:
explicit putX(std::size_t n): _n(n) {}
std::size_t getn() const {return _n;}
friend std::ostream& operator<<(std::ostream& os, const putX& obj)
{
std::size_t n = obj.getn();
for (std::size_t i = 0; i < n; ++i)
os << 'X';
return os;
}
};
int main()
{
std::cout << putX(10) << " test " << putX(10);
}
Manipulators that take no parameters can simply be implemented as
std::ostream& custom_manip(std::ostream& os) { // do something with os and return os;}
That's because basic_ostream::operator<<
has an overload that takes a pointer-to-function std::ostream& (*fp)(std::ostream&)
as its right hand side (e.g., a manipulator)
PS: The C++ Standard Library by N. Josuttis describes how manipulators/custom manipulators work in great detail, see Sec. 15.6.3 User-Defined Manipulators
来源:https://stackoverflow.com/questions/29337774/how-are-iomanip-functions-implemented