How does a language expand itself? [closed]

Answer 1

How does C++ ... suddenly get such capabilities through libraries written in C++ themselves ?

There's nothing magical about using other libraries. Libraries are simple big bags of functions that you can call.

Consider yourself writing a function like this

void addExclamation(std::string &str)
{
    str.push_back('!');
}

Now if you include that file you can write addExclamation(myVeryOwnString);. Now you might ask, "how did C++ suddenly get the capability to add exclamation points to a string?" The answer is easy: you wrote a function to do that then you called it.

So to answer your question about how C++ can get capabilities to draw windows through libraries written in C++, the answer is the same. Someone else wrote function(s) to do that, and then compiled them and gave them to you in the form of a library.

The other questions answer how the window drawing actually works, but you sounded confused about how libraries work so I wanted to address the most fundamental part of your question.

Answer 2

First, there's a little misunderstading, I think

How does C++, which previously had no syntax capable of asking the OS for a window or a way to communicate through networks

There is no syntax for doing OS operations. It's the question of semantics.

suddenly get such capabilities through libraries written in C++ themselves

Well, the operating system is writen mostly in C. You can use shared libraries (so, dll) to call the external code. Additionally, the operating system code can register system routines on syscalls* or interrupts which you can call using assembly. That shared libraries often just make that system calls for you, so you are spared using inline assembly.

Here's the nice tutorial on that: http://www.win.tue.nl/~aeb/linux/lk/lk-4.html
It's for Linux, but the principles are the same.

How the operating system is doing operations on graphic cards, network cards etc? It's a very broad thema, but mostly you need to access interrupts, ports or write some data to special memory region. Since that operations are protected, you need to call them through the operating system anyway.

Answer 3

You're right that in general, libraries cannot make anything possible that isn't already possible.

But the libraries don't have to be written in C++ in order to be usable by a C++ program. Even if they are written in C++, they may internally use other libraries not written in C++. So the fact that C++ didn't provide any way to do it doesn't prevent it from being added, so long as there is some way to do it outside of C++.

At a quite low level, some functions called by C++ (or by C) will be written in assembly, and the assembly contains the required instructions to do whatever isn't possible (or isn't easy) in C++, for example to call a system function. At that point, that system call can do anything your computer is capable of, simply because there's nothing stopping it.

Answer 4

When you try to draw something on the screen, your code calls some other piece of code which calls some other code (etc.) until finally there is a "system call", which is a special instruction that the CPU can execute. These instructions can be either written in assembly or can be written in C++ if the compiler supports their "intrinsics" (which are functions that the compiler handles "specially" by converting them into special code that the CPU can understand). Their job is to tell the operating system to do something.

When a system call happens, a function gets called that calls another function (etc.) until finally the display driver is told to draw something on the screen. At that point, the display driver looks at a particular region in physical memory which is actually not memory, but rather an address range that can be written to as if it were memory. Instead, however, writing to that address range causes the graphics hardware to intercept the memory write, and draw something on the screen.
Writing to this region of memory is something that could be coded in C++, since on the software side it's just a regular memory access. It's just that the hardware handles it differently.
So that's a really basic explanation of how it can work.

Answer 5

Good question. Every new C or C++ developer has this in mind. I am assuming a standard x86 machine for the rest of this post. If you are using Microsoft C++ compiler, open your notepad and type this (name the file Test.c)

int main(int argc, char **argv)
{
   return 0
}

And now compile this file (using developer command prompt) cl Test.c /FaTest.asm

Now open Test.asm in your notepad. What you see is the translated code - C/C++ is translated to assembler. Do you get the hint ?

_main   PROC
    push    ebp
    mov ebp, esp
    xor eax, eax
    pop ebp
    ret 0
_main   ENDP

C/C++ programs are designed to run on the metal. Which means they have access to lower level hardware which makes it easier to exploit the capabilities of the hardware. Say, I am going to write a C library getch() on a x86 machine.

Depending on the assembler I would type something this way :

_getch proc 
   xor AH, AH
   int 16h
   ;AL contains the keycode (AX is already there - so just return)
ret

I run it over with an assembler and generate a .OBJ - Name it getch.obj.

I then write a C program (I dont #include anything)

extern char getch();

void main(int, char **)
{
  getch();
}

Now name this file - GetChTest.c. Compile this file by passing getch.obj along. (Or compile individually to .obj and LINK GetChTest.Obj and getch.Obj together to produce GetChTest.exe).

Run GetChTest.exe and you would find that it waits for the keyboard input.

C/C++ programming is not just about language. To be a good C/C++ programmer you need to have a good understanding on the type of machine that it runs. You will need to know how the memory management is handled, how the registers are structured, etc., You may not need all these information for regular programming - but they would help you immensely. Apart from the basic hardware knowledge, it certainly helps if you understand how the compiler works (ie., how it translates) - which could enable you to tweak your code as necessary. It is an interesting package!

Both languages support __asm keyword which means you could mix your assembly language code too. Learning C and C++ will make you a better rounded programmer overall.

It is not necessary to always link with Assembler. I had mentioned it because I thought that would help you understand better. Mostly, most such library calls make use of system calls / APIs provided by the Operating System (the OS in turn does the hardware interaction stuff).

Answer 6

How i see the question this is actually a compiler question.

Look at it this way, you write a piece of code in Assembly(you can do it in any language) which translates your newly written language you want to call Z++ into Assembly, for simplicity lets call it a compiler (it is a compiler).

Now you give this compiler some basic functions, so that you can write int, string, arrays etc. actually you give it enough abilities so that you can write the compiler itself in Z++. and now you have a compiler for Z++ written in Z++, pretty neat right.

Whats even cooler is that now you can add abilities to that compiler using the abilities it already has, thus expanding the Z++ language with new features by using the previous features

An example, if you write enough code to draw a pixel in any color, then you can expand it using the Z++ to draw anything you want.