Say we have:
std::cout << "Something";
How exactly is this working? I just want to make sure I understand this well and, from what I've been reading, is it okay to say that basically the insertion operator inserts the string literal "Something" into the standard output stream?
But what happens after that? Where does the standard output stream lead? Can anyone explain this?
That's basically the only part I don't get: I have the string literal "Something" in the standard output stream, but where does the stream lead?
The technical details vary between the different Operating Systems, but the basics are the same:
Every program has usually 3 standard streams: out (cout), in (cin), and err (cerr) (same as out, but used for errors). Those streams are nothing on their own; they exist to be used by a third party. That third party may be, for example, the terminal. When you execute a program form the terminal, it attaches to the program streams and show their output/request their input in the terminal.
If you wanted to do the same, you could execute a command yourself from your program, and take the out/in/err streams to read or write from/to them.
You have an example of that here: How do I execute a command and get the output of the command within C++ using POSIX?
Edit: When talking about C++, remember that cout << "anything" is just syntactic sugar for the function cout.operator<<("anything"). And that function "simply" writes to the stream
so,'std' is the namespace in which is stored everything found in the standard library , so basically you are saying "hey C++, go to 'std' storage and find cout command and run it" at least I know 'std' is this. and when you are saying using namespace std; you tell the compiler "take everything that's in the std namespace and dump it in the global namespace".
I hope it helped you to understand.
I am trying to find all places in a large and old code base where certain constructors or functions are called. Specifically, these are certain constructors and member functions in the std::string class (that is, basic_string<char>). For example, suppose there is a line of code:
std::string foo(fiddle->faddle(k, 9).snark);
In this example, it is not obvious looking at this that snark may be a char *, which is what I'm interested in.
Attempts To Solve This So Far
I've looked into some of the dump features of gcc, and generated some of them, but I haven't been able to find any that tell me that the given line of code will generate a call to the string constructor taking a const char *. I've also compiled some code with -s to save the generated equivalent assembly code. But this suffers from two things: the function names are "mangled," so it's impossible to know what is being called in C++ terms; and there are no line numbers of any sort, so even finding the equivalent place in the source file would be tough.
Motivation and Background
In my project, we're porting a large, old code base from HP-UX (and their aCC C++ compiler) to RedHat Linux and gcc/g++ v.4.8.5. The HP tool chain allowed one to initialize a string with a NULL pointer, treating it as an empty string. The Gnu tools' generated code fails with some flavor of a null dereference error. So we need to find all of the potential cases of this, and remedy them. (For example, by adding code to check for NULL and using a pointer to a "" string instead.)
So if anyone out there has had to deal with the base problem and can offer other suggestions, those, too, would be welcomed.
Have you considered using static analysis?
Clang has one called clang analyzer that is extensible.
You can write a custom plugin that checks for this particular behavior by implementing a clang ast visitor that looks for string variable declarations and checks for setting it to null.
There is a manual for that here.
See also: https://github.com/facebook/facebook-clang-plugins/blob/master/analyzer/DanglingDelegateFactFinder.cpp
First I'd create a header like this:
#include <string>
class dbg_string : public std::string {
public:
using std::string::string;
dbg_string(const char*) = delete;
};
#define string dbg_string
Then modify your makefile and add "-include dbg_string.h" to cflags to force include on each source file without modification.
You could also check how is NULL defined on your platform and add specific overload for it (eg. dbg_string(int)).
You can try CppDepend and its CQLinq a powerful code query language to detect where some contructors/methods/fields/types are used.
from m in Methods where m.IsUsing ("CClassView.CClassView()") select new { m, m.NbLinesOfCode }
I'm coming from scripting languages where this js possible, but I'm not sure if this is possible in C++. I'm working with an external module, and it uses a function, which parameters are not correct, so I tried to check them, but is not as simple as in JavaScript.
To check a parameter value, how can I do it? A simple cout gives me errors about types, and the same if I try to convert them to strings.
Is possible see the parameter value as in JS using a console.log(fooParameter); or something similar?
Thank's advanced!
You can try to use breakpoints in your IDE in order to pause the program when callstack reaches that point and see variables.
You can also overload the << operator in order to write to std::cout your parameter type if it's not a predefined one (string, int, etc).
c++ is an explicitly typed language, so you should have full control over what type is being passed.
However, you can print the variable type during runtime with:
#include <typeinfo>
// …
std::cout << typeid(fooParameter).name() << '\n';
Hope this helps!
Is there a way out to call a function directly from the what the user inputs ?
For example : If the user inputs greet the function named greet is called.
I don't want any cases or comparison for the call to generate.
#include <iostream>
#include<string>
using namespace std;
void nameOfTheFunction(); // prototype
int main() {
string nameOfTheFunction;
getline(cin,nameOfTheFunction); // enter the name of Function
string newString = nameOfTheFunction + "()"; // !!!
cout << newString;
// now call the function nameOfTheFunction
}
void nameOfTheFunction() {
cout << "hello";
}
And is there a concept of generating the function at run time ?
You mean run time function generation ??
NO.
But you can use a map if you already know which all strings a user might give as input (i.e you are limiting the inputs).
For the above you can probably use std::map < std::string, boost::function <... > >
Check boost::function HERE
In short, no this isn't possible. Names in C++ get turned into memory offsets (addresses), and then the names are discarded**. At runtime C++ has no knowledge of the function or method names it's actually running.
** If debug symbols are compiled in, then the symbols are there, but impractical to get access to.
Generating a function at runtime has a lot of drawbacks (if it is possible at all) and there is generally no good reason to do it in a language like C++. You should leave that to scripting languages (like Perl or Python), many offer a eval() function that can interpret a string like script code and execute it.
If you really, really need to do have something like eval() in a compiled language such as C++, you have a few options:
Define your own scripting language and write a parser/interpreter for it (lots of work)
Define a very simple imperative or math language that can be easily parsed and evaluated using well-known design patterns (like Interpreter)
Use an existing scripting language that can be easily integrated into your code through a library (example: Lua)
Stuff the strings of code you want to execute at runtime through an external interpreter or compiler and execute them through the operating system or load them into your program using dlopen/LoadLibrary/etc.
(3.) is probably the easiest and best approach. If you want to keep external dependencies to a minimum or if you need direct access to functionality and state inside your main program, I suggest you should go for (2.) Note that you can have callbacks into your own code in that case, so calling native functions from the script is not a problem. See here for a tutorial
If you can opt for a language like Java or C#, there's also the option to use the compiler built into the runtime itself. Have a look here for how to do this in Java
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Closed 13 years ago.
I'm a novice at programming although I've been teaching myself Python for about a year and I studied C# some time ago.
This month I started C++ programming courses at my university and I just have to ask; "why is the C++ code so complicated?"
Writing "Hello world." in Python is as simple as "print 'Hello world.'" but in C++ it's:
# include <iostream>
using namespace std;
int main ()
{
cout << "Hello world.";
return 0;
}
I know there is probably a good reason for all of this but, why...
... do you have to include the <iostream> everytime? Do you ever not need it?
... same question for the standard library, when do you not need std::*?
... is the "main" part a function? Do you ever call the main function? Why is it an integer? Why does C++ need to have a main function but Python doesn't?
... do you need "std::cout << "? Isn't that needlessly long and complicated compared to Python?
... do you need to return 0 even when you are never going to use it?
This is probably because I'm learning such basic C++ but every program I've made so far looks like this, so I have to retype the same code over and over again. Isn't that redundant? Couldn't the compiler just input this code itself, since it's always the same (i.e. afaik you always include <iostream>, std, int main, return 0)
C++ is a more low-level language that executes without the context of an interpreter. As such, it has many different design choices than does Python, because C++ has no environment which it can rely on to manage information like types and memory. C++ can be used to write an operating system kernel where there is no code running on the machine except for the program itself, which means that the language (some library facilities are not available for so-called freestanding implementations) must be self-contained. This is why C++ has no equivalent to Python's eval, nor a means of determining members, etc. of a class, nor other features that require an execution environment (or a massive overhead in the program itself instead of such an environment)
For your individual questions:
do you have to include the <iostream> everytime? Do you ever not need it?
#include <iostream> is the directive that imports the <iostream> header into your program. <iostream> contains the standard input/output objects - in particular, cout. If you aren't using standard I/O objects (for instance, you use only file I/O, or your program uses a GUI library, or are writing an operating system kernel), you do not need <iostream>
same question for the standard library, when do you not need std::*?
std is the namespace containing all of the standard library. using namespace std; is sort of like from std import *, whereas a #include directive is (in this regard) more like a barebones import std statement. (in actual fact, the mechanism is rather different, because C++ does not use using namespace std; to automatically lookup objects in std; the using-directive only imports the names into the global namespace.)
I'll note here that using-directives (using namespace) are frequently frowned upon in C++ code, as they import a lot of names and can cause name clashes. using-declarations (using std::cout;) are preferred when possible, as is limiting the scope of a using-directive (for instance, to one function or to one source file). Don't ever put using namespace in a header without good reason.
is the "main" part a function? Do you ever call the main function? Why is it an integer? Why does C++ need to have a main function but Python doesn't?
main is the entry point to the program - where execution starts. In Python, the __main__ module serves the same purpose. C++ does not execute code outside a defined function like Python does, so its entry point is a function rather than a module.
do you need "std::cout << "? Isn't that needlessly long and complicated compared to Python?
std::cout is only needed if you don't import the cout name into the global namespace, either by a using-directive (using namespace std;) or by a using-declaration (using std::cout). In this regard, it is once again much like the distinction between Python's import std and from std import * or from std import cout.
The << is an overloaded operator for standard stream objects. cout << value calls cout's function to output value. Python needs no such extra code because print is built into the language; this does not make sense for C++, where there may not even be an operating system, much less an I/O library.
do you need to return 0 even when you are never going to use it?
No. main (and no other function) has an implicit return 0; at the end. The return value of main (or, if the exit function is called, the value passed to it) is passed back to the operating system as the exit code. 0 indicates the program successfully executed - that it encountered no errors, etc. If an error is encountered, a non-zero value should be returned (or passed to exit).
In response to your questions at the end of the post, it can be summed up with the philosophy of C++:
You don't pay for what you don't use.
You don't always need to use stdin or stdout (Windows/GUI apps?), nor will you always be using the STL, nor will everything you write necessarily use the standard main (winAPI) etc. As a previous poster said, C++ is lower level than Python. You will be exposed to more of the details, which offers you more control over what you're doing.
... do you have to include the
everytime? Do you ever not
need it?
You don't need it if you're not going to use iostreams in that module. In larger programs, few modules do any actual IO directly, and so few actually need to use iostreams.
Turning the question around: in python you need to import sys and/or os in most non-trivial programs. Why?
... same question for the standard
library, when do you not need std::*?
You can have the using line or you can use the std:: prefix. This is very similar to the choice python gives you of either saying "from sys import *" or "import sys" and then having to prefix things with "sys.". In python you have to say "sys.stdout". Is "std::cout" really any worse?
... is the "main" part a function? Do
you ever call the main function? Why
is it an integer? Why does C++ need to
have a main function but Python
doesn't?
Yes, main is a function. Typically you wouldn't call main yourself. The name "main" is reserved for the entry-point of your program. It returns an integer because the value returned is used as the status code of your program. In Python you can use sys.exit if you want to return a non-zero status code.
Python doesn't have the same convention because with Python you can have code in a module not in a function. This code is executed when you load the module. Interestingly, many people feel it is bad style to have code at the top-level of a module and will instead create a main function by doing something like this:
def main(argv):
# program goes here
return 0
if __name__ == '__main__':
sys.exit(main(sys.argv))
Also, in Python you tell the interpreter with module is the "main" module when you run it. eg: "python foo.py". In C, the "main" module is (effectively) the one with a function called main. (If there are multiple modules with a main function, it's a linker error.)
... do you need "std::cout << "? Isn't
that needlessly long and complicated
compared to Python?
The equivalent in Python is actually "sys.stdout.write(...)". Python's print statement is a special-case short-hand.
That said, many people do feel the iostreams convention of using bit-shifting operators for IO was a bad idea. Ironically, Python seems to have been "inspired" by this syntax. If you want to use print to write to somewhere other than stdout you can say:
print >>file, "Hello"
... do you need to return 0 even when
you are never going to use it?
You aren't going to use it, but your program will. As mentioned earlier, the value you return is the status code of your program.
Aside: I actually do feel that C++ is overcomplicated, but not because of any of the points you mention. All of the differences you mention go away (in the sense that you need just as much complexity in Python) once you start writing non-trivial programs that have multiple modules and do more than just writing to stdout.
You include <iostream> when you want to output things to the console. Since printing "Hello world" involves console output, you need iostream.
The main function is called by the operating system, basically. It gets called with the command-line arguments passed to the program. It returns an integer because the program must return an error code to the operating system (this is the standard way for determining if the last command was successful).
You can always use printf("hello world"); instead of std::cout << "hello world"; if you want to go C style. It's a bit quicker to write and lets you do formatted output.
You return 0 from main to indicate that the program executed successfully.
The compiler does not automatically include all the standard libraries and use namespace std because sometimes name collisions can result between your code and library code that you may not actually need at all. You don't always need all the libraries. Likewise, sometimes you are using a different main routine (Windows development comes to mind with its own, different WinMain starting function). The compiler also does not automatically return 0 because sometimes the program needs to indicate that it completed unsuccessfully.
There are good reasons for all these things. C++ is a very broad language it is used for everything from small embedded systems to giant applications built by 100s of programmers. The use case of a guy building a small program to run on a desktop is by no means the only one. So sometimes you are building library components. In that case no main(). Sometimes you are working on a tiny system with no standard library. In that case no std. Sometimes you want to build a Unix tool that works with other Unix text tools and signals its completion status with an int returned from main().
In other words the things you complain about are boilerplate to you. But they are vital details that vary to other users of the language.
This reminds me of The Evolution of a Programmer. Some of the languages and technologies demonstrated are a bit dated now, but you should get the general idea. :)
One of the reasons C++ is rather complicated is because it was designed to address problems that crop up in large programs. At the time C++ was created as AT&T, their biggest C program was about 10 million lines of code. At that scale, C doesn't function very well. C++ addresses many of the problems you get with that kind of program.
With that said, it's also possible to answer the original questions:
You would include <iostream> where it's needed. If you've got 10.000 C++ files, it's quite common that less than 1000, sometimes less than 100 will produce user-visible output.
A statement like print "Hello, world" assumes that there is a default output, but makes it hard to generalize. The cout << "Hello, world" form makes it explicit where the output goes, but the same form also allows cerr << "Goodbye, world" and MyTmpFile << "Starting phase #" << i
The standard library is in the std:: namespace. My 10.000 files will be in an additional 25 namespaces.
main is an oddity in many ways, being the startup function.
Baldur:
You don't always need <iostream>. The only things that you will always need are:
A main function (or a WinMain, if you're writing Win32 apps).
Variables, functions, operators, language constructs (if, while, etc.).
The ability to include functionality from libraries into your program.
Everything else is application-specific.
As other posters say, the return value of the main function is an error code1. If main returns 0, be happy: everything worked OK!
1This is useful when you write programs that "communicate" with other programs. The most simple way that a program can "tell" another whether it executed properly is using an error code.
As people have said, the simple answer is that they're different languages, with different goals. To answer your specific questions...
... do you have to include the <iostream> everytime? Do you ever not need it?
<iostream> is one of the header files for iostreams, the part of the C++ standard library responsible for input/output; in this instance, you need it to gain access to std::cout. If you're not doing I/O operations in a source file, you don't need to include it -- for example, most files containing class definitions probably won't need <iostream>.
... same question for the standard library, when do you not need std::*?
std is the name of namespace containing classes in the standard library; it's there to avoid name collisions. Python has packages and modules to do this.
You can use the using statement to bring items from another namespace into your current scope, see this FAQ entry for an example (and an explanation of why it's bad to blindly bring all of std into scope!).
... why is the "main" part a function? Do you ever call the main function? Why is it an integer? Why does C++ need to have a main function but Python doesn't?
Executable statements in C++ have to be contained within a function, and the main function is defined as where execution begins. In Python, executable statements can be placed at the top-level of a file, and execution is defined to .
You can call main() if you wish -- it's just a function, after all -- but there's not often a reason to do this. Behind the scenes, most implementations of C++ call main() for you once some startup housekeeping has been done by the runtime library.
The return value of main() is returned back to the operating system. This stems from C and UNIX, in which application programs are required to provide a 1-byte exit status code, and returning that value from main() is a clear way of expressing this.
... why do you need "std::cout << "? Isn't that needlessly long and complicated compared to Python?
This is just a design difference. iostreams is a fairly complex beast with lots of features, and one of the side-effects of this is that the syntax is a bit ugly for simple tasks at times.
... why do you need to return 0 even when you are never going to use it?
You do use it; this is the value returned to the operating system as the exit status of the program.
Python is high-level language. C++ is middle-level language.