C++ example program that came with a lib... how does this even compile?

[Red Squirrel]

/*
 * Copyright (C) 2007,2008   Alex Shulgin
 *
 * This file is part of png++ the C++ wrapper for libpng.  PNG++ is free
 * software; the exact copying conditions are as follows:
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 *
 * 3. The name of the author may not be used to endorse or promote products
 * derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN
 * NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
#include <cstdlib>
#include <iostream>
#include <ostream>

/* #include <png.hpp> */
#include "../png.hpp"

class pixel_generator  : public png::generator< png::gray_pixel_1, pixel_generator >
{
public:
    pixel_generator(size_t width, size_t height)   : png::generator< png::gray_pixel_1, pixel_generator >(width, height),  m_row(width)
    {
        for (size_t i = 0; i < m_row.size(); ++i)
        {
            m_row[i] = i > m_row.size() / 2 ? 1 : 0;
        }
    }

    png::byte* get_next_row(size_t /*pos*/)
    {
        size_t i = std::rand() % m_row.size();
        size_t j = std::rand() % m_row.size();
        png::gray_pixel_1 t = m_row[i];
        m_row[i] = m_row[j];
        m_row[j] = t;
        return reinterpret_cast< png::byte* >(row_traits::get_data(m_row));
    }

private:
    typedef png::packed_pixel_row< png::gray_pixel_1 > row;
	typedef png::row_traits< row > row_traits;
	row m_row;
};

int main()
try
{
    size_t const width = 512;
    size_t const height = 512;

    std::ofstream file("generated.png", std::ios::binary);
    pixel_generator generator(width, height);
    generator.write(file);
}
catch (std::exception const& error)
{
    std::cerr << "pixel_generator: " << error.what() << std::endl;
    return EXIT_FAILURE;
}

There's no main function. Well, there's int main() but no brackets to encapsulate any code into it.

How does this compile and where does the program actually start? And yes, it compiles, and runs! Supposedly it should compile with the make command but that fails but if you do this it works:

g++ -o pixel_generator pixel_generator.cpp  `libpng-config --ldflags

 

[Markbnj]

It's been awhile since I wrote C, but I believe brackets are optional in that case. The try/catch is presumably treated as a single statement by the compiler. At least that is my best guess .

 

[Red Squirrel]

Interesting. What does it return though? First time I see something like this.

 

[Markbnj]

Interesting. What does it return though? First time I see something like this.

The return value is optional in main(). It's actually called from a wrapper in the preamble code for the executable (that's a less than technical answer - been a long time) and I assume it substitutes some default return value if one isn't provided when main() exits.

Interesting, though, I seem to recall you had to use "void main" instead of "int main" if you didn't return a value.

 

[Venix]

It's a function-level try block. It doesn't offer any particular value here other than removing unnecessary braces, but it's the only way to catch exceptions in a C++ constructor initialization list.

According to the C++ standard, int main() contains an implicit "return 0;" at the end of the function.

 

[Red Squirrel]

Can something like this be used as a global exception catcher? I've used some libraries that throw exceptions and that crashes the whole program if I don't explicitly use a try/catch block (which would be very messy to do each time) but having a global one that at least outputs info to a crash log would definitely be useful. Idealy I usually try to avoid situations where exceptions can be thrown and put them in some kind of wrapper class that returns a value instead of crashing but sometimes it's possible to miss something when using unknown/new libraries.