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I'm a newcomer to C++, from C#. I wrote a program that prints an ASCII checkerboard with a side length of the number passed to it (link to original Code Golf question).

stdafx.h

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

#include <iostream>
#include <sstream>
#include <string>

Checkers.cpp

#include "stdafx.h"

using std::cin;
using std::cout;
using std::endl;
using std::stoi;
using std::string;
using std::stringstream;

string header(int num);

template <bool white>
string body(const string &head, int num);

template <bool white>
stringstream& append_body(stringstream &stream, int rem, int approx);

int _tmain(int argc, _TCHAR* argv[])
{
    int num;

    if (argc == 1 || ((num = stoi(argv[1])) < 0))
    {
        cout << "Usage: ./Checkers [nonnegative int]" << endl;
        return 1;
    }

    switch (num)
    {
    case 0:
        return 0;
    case 1:
        cout << "+--+" << endl << "|  |" << endl << \
            "|  |" << endl << "+--+" << endl;
        return 0;
    }

    int rem = num % 2;
    int approx = num - rem;

    const string& head = header(num);
    string& wtail = body<true>(head, num);
    string& btail = body<false>(head, num);

    string& block = wtail + btail;

    stringstream stream;

    for (int i = 0; i < approx / 2; i++)
        cout << block;

    if (rem == 1)
        cout << wtail;

    cout << head << endl;

#ifdef _DEBUG
    cin.get();
#endif
}

string header(int num)
{
    string result;

    for (int i = 0; i < num; i++)
        result += "+--";

    return result + '+';
}

template <bool white>
string body(const string &head, int num)
{
    int rem = num % 2;
    int approx = num - rem;

    stringstream stream;
    stream << head << endl;

    append_body<white>(stream, rem, approx) << endl;
    append_body<white>(stream, rem, approx) << endl;

    return stream.str();
}

template <bool white>
stringstream& append_body(stringstream &stream, int rem, int approx)
{
    const char *const block =
        white ? "|  |##" : "|##|  ";

    for (int i = 0; i < approx / 2; i++)
        stream << block;

    stream << '|';

    if (rem == 1)
        stream << (white ? "  |" : "##|");

    return stream; // not a local so this is OK
}

Questions:

  • I intentionally had a little fun optimizing prematurely here, i.e. generating the header ("+--+") and body ("|##| |") strings only once instead of every loop. What could I do to increase readability without getting rid of each-loop optimizations like this?

  • When is it okay to use non-type template parameters, like I did in body? This means that you always have to rewrite your code when you want to offer a non-template alternative, correct?

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  • To appreciate a power of non-type templates, try to implement body and append_body in plain C. You will see immediately why your approach is an abuse of templates.

  • Most important

    body() returns a temporary object:

        stringstream stream;
        ...
        return stream.str();
    

    and clang expectedly complains:

    error: non-const lvalue reference to type
      'basic_string<[3 * ...]>' cannot bind to a temporary of type
      'basic_string<[3 * ...]>'
    string& wtail = body<true>(head, num);
            ^       ~~~~~~~~~~~~~~~~~~~~~
    
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