Question

I’d like to implement a custom manipulator for ostream to do some manipulation on the next item being inserted into the stream. For example, let’s say I have a custom manipulator quote:

std::ostringstream os; std::string name('Joe'); os << 'SELECT * FROM customers WHERE name = ' << quote << name;   

The manipulator quote will quote name to produce:

SELECT * FROM customers WHERE name = 'Joe' 

How do I go about accomplishing that? Thanks.

Answer 1

It’s particularly difficult to add a manipulator to a C++ stream, as one has no control of how the manipulator is used. One can imbue a new locale into a stream, which has a facet installed that controls how numbers are printed – but not how strings are output. And then the problem would still be how to store the quoting state safely into the stream.

Strings are output using an operator defined in the std namespace. If you want to change the way those are printed, yet keeping the look of manipulators, you can create a proxy class:

namespace quoting { struct quoting_proxy {     explicit quoting_proxy(std::ostream & os):os(os){}      template<typename Rhs>     friend std::ostream & operator<<(quoting_proxy const& q,                                       Rhs const& rhs) {         return q.os << rhs;     }      friend std::ostream & operator<<(quoting_proxy const& q,                                       std::string const& rhs) {         return q.os << ''' << rhs << ''';     }      friend std::ostream & operator<<(quoting_proxy const& q,                                       char const* rhs) {         return q.os << ''' << rhs << ''';     } private:     std::ostream & os; };  struct quoting_creator { } quote; quoting_proxy operator<<(std::ostream & os, quoting_creator) {     return quoting_proxy(os); } }  int main() {     std::cout << quoting::quote << 'hello' << std::endl;  } 

Which would be suitable to be used for ostream. If you want to generalize, you can make it a template too and also accept basic_stream instead of plain string. It has different behaviors to standard manipulators in some cases. Because it works by returning the proxy object, it will not work for cases like

std::cout << quoting::quote;  std::cout << 'hello';