How to output two versions of a string, one with escape character and the other not, in C++?

Question

I have one chance to define the string, lets say

string s = "abc\"def\\hhh\"i";

After this definition, I want to output (using ofstream to write to a text file) two versions of this string afterwards. The first one is the output of s by default:

abc"def\hhh"i

The second one I want is:

abc\"def\\hhh\"i

I am writing a sort of "recursive" definition, defining another string with extra escape characters is not a solution. I also looked up raw string, but it can only output the second not the first, and it is a feature for c++11, which is too new for some computers to compile.

How can I output the second version of the string without using c++11? If I have to use c++11, how to avoid defining the string twice?

May be a XY poblem. Why and for what purpose you need that exactly? — πάντα ῥεῖ, Mar 05 '17 at 17:22
You can provide a macro that turns the initial literal into a raw character literal as needed. — πάντα ῥεῖ, Mar 05 '17 at 17:24
@Jarod42 [Cool](http://en.cppreference.com/w/cpp/io/manip/quoted). — πάντα ῥεῖ, Mar 05 '17 at 17:25
@Jarod42 thanks for you advice, but it is introduced in c++14, which is even newer than c++11:( — Simple Mistake, Mar 05 '17 at 17:26
@Jarod42, Unfortunately, it handles only one character to escape, so it would need multiple calls, plus a stream, and at that point, you might as well use `replace` (or I guess Boost's `replace_all` since neither `replace` in the standard library handles all and multiple character replacements). — chris, Mar 05 '17 at 17:29
BTW, you may write your own function, which replaces first `\` by `\\` and then `"` by `\"`. — Jarod42, Mar 05 '17 at 17:32
@zett42, Sure, but you use it like `out << quoted(...)` or `in >> quoted(...)`. — chris, Mar 05 '17 at 18:44
@chris ...and? You can write `std::string s; out << quoted(s)` or `std::string s; in >> quoted(s)`. Anyway this discussion doesn't help OP much as he don't want to use C++11 (quoted is C++14). — zett42, Mar 05 '17 at 18:52
@zett42, Which part of my comment were you addressing? I said you need a stream, and `in`/`out` are streams regardless of which overload is used. — chris, Mar 05 '17 at 18:56
@chris First _it handles only one character to escape, so it would need multiple calls_ . Additionally OP explicitly asked for stream, so `std::quoted` would be perfect. — zett42, Mar 05 '17 at 19:00
@zett42, I see. That part was referencing the fact that you can't give it a list of characters counting as quotation marks, rather than trying to say it wouldn't replace more than one occurrence. Anyway, I missed the stream part of the question, so maybe it's too bad this isn't C++14. — chris, Mar 05 '17 at 19:04

Peter Lenkefi · Accepted Answer · 2017-03-05T18:24:58.487

It is very simple to write such functionality:

std::string to_quoted(std::string const& src) {
    std::string result;
    result.reserve(src.size());  // Not necessary but makes the code more efficient (mentioned in comments)
    for (std::size_t i = 0; i < src.length(); i++) {
        switch (src[i]) {
        case '"': result += "\\""; break;
        case '\\': result += "\\\\"; break;
        // ...
        default: result += src[i];
        }
    }
    return result;
}

There may be better solutions, I think this is the simplest and quickest one. I don't understand what you mean by "defining another string", maybe you mean constructing another string is disallowed, in that case just output to the stream instead of concatenating characters.

A little suggestion: add `result.reserve(src.size())` after creating the `result` variable to reduce the number of reallocations. For the case that there is nothing to escape, there would only be a single allocation necessary and propably only one additional allocation if there is something to escape (assuming that std::string doubles the size of the allocated buffer on overflow). — zett42, Mar 05 '17 at 18:14

zett42 · Answer 2 · 2017-03-05T19:47:19.130

I tend to write something like this as template with range input and iterator output. This provides much flexibility as you can output to a stream, another string or anything else that you could wrap into an output iterator, all using the same function.

Input doesn't even have to be a std::string, it could be a std::vector, a simple array or any type for which an overload of begin() and end() is provided (requirements of range-for loop).

Another advantage compared to simply returning an std::string from the function is that you don't have to create a temporary string for the result which avoids memory allocations which should improve performance.

#include <iostream>
#include <string>
#include <iterator>

template< typename Range, typename OutputIterator >
OutputIterator copy_escaped( const Range& in, OutputIterator out ){
    for( const auto& c : in ){
        switch( c ){
            case '"': 
                *out++ = '\\';
                *out++ = '"';
                break;
            case '\\':
                *out++ = '\\';
                *out++ = '\\';
                break;
            case '\n':
                *out++ = '\\';
                *out++ = 'n';
                break;
            case '\r':
                *out++ = '\\';
                *out++ = 'r';
                break;
            case '\t':
                *out++ = '\\';
                *out++ = 't';
                break;
            // Could add more stuff to escape here
            // case ...:
            default:
                *out++ = c;
        }
    }
    return out;
}

You could easily extend the function to escape additional characters.

Usage examples:

int main()
{
    std::string s = "abc\"def\\hhh\"i";

    // output normal
    std::cout << s << std::endl;

    // output escaped
    copy_escaped( s, std::ostream_iterator<char>( std::cout ) );
    std::cout << std::endl;

    // output escaped to other string
    std::string escaped_s;
    escaped_s.reserve( s.size() );  // not required but improves performance
    copy_escaped( s, back_inserter( escaped_s ) );
    std::cout << escaped_s << std::endl;
}

Live demo.

Damnit, just noted that this is C++11 too: `for( const auto& c : in )` ... so used to this good stuff already ;-). — zett42, Mar 05 '17 at 19:57
[This](http://coliru.stacked-crooked.com/a/a6cbe700ff65e36a) would be an implementation that doesn't require C++11. — zett42, Mar 05 '17 at 20:13
[Another implementation](http://coliru.stacked-crooked.com/a/a808bc9ea59a4ec8) without C++11, but requires [boost.range library](http://www.boost.org/doc/libs/release/libs/range/). — zett42, Mar 05 '17 at 20:15

How to output two versions of a string, one with escape character and the other not, in C++?

2 Answers2