How to add indention to the stream operator

Question

In our project we use the c++ stream operator (<<) in our object model to print out an easy readible format of the data. A simplified example is this:

Answer 1

This can be done using a custom stream-manipulator that stores the desired indentation-level in a word of the internal extensible array of the stream. You can request such a word using the function ios_base::xalloc. This function will give you the index of your word. You can access it using ios_base::iword. One way to implement that would be this:

struct indent {
    indent(int level) : level(level) {}
private:
    friend std::ostream& operator<<(std::ostream& stream, const indent& val);

    int level;
};

std::ostream& operator<<(std::ostream& stream, const indent& val) {
    for(int i = 0; i < val.level; i++) {
        stream << " ";
    }
    return stream;
}

std::ostream& operator<<(std::ostream & oStream, const OwnClass& iOwnClass) {
    oStream << indent(oStream.iword(index)) << "[SomeMember1: " << 
               iOwnClass._ownMember1 << "]\n";
    oStream << indent(oStream.iword(index)) << "[SomeMember2: " << 
               iOwnClass._ownMember2 << "]\n";
}

You'd have to figure out where to store the index. This effectively allows you to add custom state to the stream (note that this would not be thread-safe out-of-the-box). Every function that wants indentation should add the requested indentation to the stream, and subtract it again when it is done. You could make sure this always happen by using a guard to add/subtract the desired indent (IMHO this is more elegant than using a manipulator):

class indent_guard {
public:
    indent_guard(int level, std::ostream& stream, int index) 
    : level(level),
      stream(stream),
      index(index)
    {
        stream.iword(index) += level;
    }

    ~indent_guard() {
        stream.iword(index) -= level;
    }

 private:
     int level;
     std::ostream& stream;
     int index;
};

You could use it like this:

void some_func() {
    indent_guard(2, std::cout, index);

    // all output inside this function will be indented by 2 spaces

    some_func(); // recursive call - output will be indented by 4 spaces

    // here it will be 2 spaces again
}