Stream Precision Modifier for strings

Lets say I have string foo("lorem ipsum"). Now i want to print a maximum number of characters from foo, let's say x which is guaranteed to be smaller than foo.size().

This is very continent in printf:

printf("%.*s", x, foo.data());

But the only way I can find to do this in a stream is to construct a temporary string:

cout << string(foo, x);

Is there a manipulator that will let me set the precision for streams, or is constructing a temporary string all that's available to me?

Solution

There is no "stream manipulator" to cut off strings after a specified width.

What you're looking for is basically string_view⁽¹⁾, that is some light weight substring wrapper which is ostream-able.

The following line would print the first x characters of foo without copying it to a temporary string, as long as the guarantee you mention (x >= foo.size()) holds:

cout << string_view(foo.data(), x);

In case the guarantee doesn't hold anymore, use setw to fill the field with whitespaces when the string is shorter, in order to print a fixed field length of x characters. The string_view constructor needs a properly bound-limited length since it doesn't know the size of the "real" std::string object, thus we use min to limit x to the length of the string:

cout << setw(x) << string_view(foo.data(), min(x, foo.size()));

If you don't want to or can't use string_view, you can write your own light weight wrapper just for the purpose of printing a substring with ostream.

class substr {
     const std::string & s;
     std::size_t len;

     friend std::ostream& operator<<(std::ostream& os, const substr &ss) {
         std::copy(ss.s.begin(), ss.s.begin() + std::min(ss.len, ss.s.size()),
                   std::ostream_iterator<char>(os));
         return os;
     }
public:
     substr(const std::string & s, std::size_t len) : s(s), len(len) {}
};

The usage is then:

cout << substr(foo, x);

Live Example

^{(1) This class is currently experimental and not yet in the standard. As far as I know, it probably will be in C++17 and is available as std::experimental::string_view in <experimental/string_view> since g++ 4.9 when using -std=c++1y or 17.}