C++ Getting started with C++ Hello World
Example
This program prints Hello World! to the standard output stream:
#include <iostream>
int main()
{
std::cout << "Hello World!" << std::endl;
}
See it live on Coliru.
Analysis
Let's examine each part of this code in detail:
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#include <iostream>is a preprocessor directive that includes the content of the standard C++ header fileiostream.iostreamis a standard library header file that contains definitions of the standard input and output streams. These definitions are included in thestdnamespace, explained below.The standard input/output (I/O) streams provide ways for programs to get input from and output to an external system -- usually the terminal.
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int main() { ... }defines a new function namedmain. By convention, themainfunction is called upon execution of the program. There must be only onemainfunction in a C++ program, and it must always return a number of theinttype.Here, the
intis what is called the function's return type. The value returned by themainfunction is an exit code.By convention, a program exit code of
0orEXIT_SUCCESSis interpreted as success by a system that executes the program. Any other return code is associated with an error.If no
returnstatement is present, themainfunction (and thus, the program itself) returns0by default. In this example, we don't need to explicitly writereturn 0;.All other functions, except those that return the
voidtype, must explicitly return a value according to their return type, or else must not return at all.
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std::cout << "Hello World!" << std::endl;prints "Hello World!" to the standard output stream:-
stdis a namespace, and::is the scope resolution operator that allows look-ups for objects by name within a namespace.There are many namespaces. Here, we use
::to show we want to usecoutfrom thestdnamespace. For more information refer to Scope Resolution Operator - Microsoft Documentation. -
std::coutis the standard output stream object, defined iniostream, and it prints to the standard output (stdout). -
<<is, in this context, the stream insertion operator, so called because it inserts an object into the stream object.The standard library defines the
<<operator to perform data insertion for certain data types into output streams.stream << contentinsertscontentinto the stream and returns the same, but updated stream. This allows stream insertions to be chained:std::cout << "Foo" << " Bar";prints "FooBar" to the console. -
"Hello World!"is a character string literal, or a "text literal." The stream insertion operator for character string literals is defined in fileiostream. -
std::endlis a special I/O stream manipulator object, also defined in fileiostream. Inserting a manipulator into a stream changes the state of the stream.The stream manipulator
std::endldoes two things: first it inserts the end-of-line character and then it flushes the stream buffer to force the text to show up on the console. This ensures that the data inserted into the stream actually appear on your console. (Stream data is usually stored in a buffer and then "flushed" in batches unless you force a flush immediately.)An alternate method that avoids the flush is:
std::cout << "Hello World!\n";where
\nis the character escape sequence for the newline character. -
The semicolon (
;) notifies the compiler that a statement has ended. All C++ statements and class definitions require an ending/terminating semicolon.
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