C++ cout: how to print with std::cout

In C++, cout prints to the screen: std::cout << "Hello"; sends the text to standard output, and each further << adds one more piece. It is declared in the header <iostream>, a library file you #include, and lives in the namespace std, the group that holds the library’s names, which is why it is written std::cout. End a line with '\n'.

#include <iostream>

int main() {
    std::cout << "Hello, world!\n";
}

It prints Hello, world! and ends the line. Everything on this page is C++17 or older, except std::println in the last section, which needs C++23 (Since When lists the compiler releases for each standard). The samples were built with MSVC 19.44, Clang 19.1 (MSVC STL), GCC 16.2 and Clang 23.1 (libc++), and all four printed the same.

How do you print variables and numbers with cout?

Give each one its own <<: text in double quotes, a variable, a number or a calculation, in the order they should appear. Here, a shopping total:

#include <iostream>
#include <string>

int main() {
    std::string item = "coffee";
    int count = 3;
    double price = 4.5;

    std::cout << "Item: " << item << '\n';            // a std::string
    std::cout << "Price: " << price << '\n';          // a double
    std::cout << count << " x " << item << '\n';      // an int, text
    std::cout << "Total: " << price * count << '\n';  // a calculation
}

All four printed:

Item: coffee
Price: 4.5
3 x coffee
Total: 13.5

std::cout is a stream, an object you send output into, and << is the insertion operator. Each << sends the value on its right into the stream on its left, then hands the stream back, so the next << sends to std::cout again. That is why one statement can chain as many as you need:

cout adds no spaces or line breaks of its own: the spaces in " x " are the ones you see. By default a double prints without trailing zeros, so 4.5 times 3 printed 13.5; formatting numbers, below, fixes the decimals. A type of your own prints the same way once it has an operator<< (C++ operator overloading shows how).

How do you start a new line: '\n' or std::endl?

Write '\n': it ends the line. std::endl ends it too, and also flushes the output: it makes the stream write out at once whatever it has collected so far, which you rarely need.

std::cout << "coffee" << '\n';
std::cout << "tea\n";
std::cout << "milk" << std::endl;  // also flushes the output

All three ended a line: the program printed coffee, tea and milk on three lines, so "\n" inside the text works as well as '\n' after it. A flush on every line is extra work in a loop that prints many of them; when output must appear at once, std::flush flushes without a newline. What the flush does, and std::endl vs '\n' has the longer answer.

Why does cout not work without std::?

Because cout lives in the namespace std, and the compiler does not find the bare name outside it. With <iostream> included and no std:::

cout << "Hello, world!\n";  // error: cout lives in std

GCC stopped with:

04-no-std-fail.cpp:5:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?

Clang says the same in other words; MSVC says only 'cout': undeclared identifier. cout and std::cout are the same object, and the short name works only after a using. Any of three fixes works:

• std::cout every time, as on this page.
• using std::cout; once, after which cout alone means std::cout.
• using namespace std;, which lets you leave std:: off every name in the library, with the name clashes that using namespace std explains.

The second names only what you use:

#include <iostream>

using std::cout;  // from here on, cout means std::cout

int main() {
    cout << "Hello, world!\n";
}

It printed Hello, world! on all four. The other error a first program meets is a missing #include <iostream>, with std::cout written in full. The compilers word that one differently: GCC and MSVC say cout is not a member of std, and Clang that there is no member cout in std or, in a file that includes nothing, that std itself is undeclared. The fix is the same: add the include at the top.

How do you fix “undefined reference to std::cout”?

Build with g++, not gcc. The message is a linker error: the code compiled, but the linker, the step that joins your compiled code with the libraries it uses, was not given the C++ standard library, where std::cout is defined. gcc compiles a .cpp file as C++ but does not add that library; g++ adds it by itself.

Built as C++ by GCC 16.2’s gcc on Compiler Explorer, which names the file <source>, the first example compiled and then failed to link. Two of the four lines it printed:

<source>:4:(.text+0xa): undefined reference to `std::cout'
collect2: error: ld returned 1 exit status

Adding -lstdc++, the flag that names GCC’s C++ library, made the same build link and print Hello, world!. So build with g++, or clang++ with Clang.

How do you format numbers with cout?

Send manipulators, values that change how the stream prints what comes after them: std::fixed with std::setprecision(2) gives two decimals, and std::setw(10) pads the next item to 10 characters. std::setprecision and std::setw need #include <iomanip>; std::fixed comes with <iostream>.

double total = price * count;

std::cout << total << '\n';                          // plain
std::cout << std::setprecision(3) << total << '\n';  // 3 digits
std::cout << std::fixed << std::setprecision(2);     // 2 decimals
std::cout << total << '\n';
std::cout << price << '\n';                          // still on
std::cout << count << " x " << item << ": " << total << '\n';

std::cout << '[' << std::setw(10) << item << ']'     // padded
          << '[' << item << "]\n";                   // not padded
std::cout << '[' << std::left << std::setw(10) << item << "]\n";

All four printed:

13.5
13.5
13.50
4.50
3 x coffee: 13.50
[    coffee][coffee]
[coffee    ]

std::setprecision(3) alone counts significant digits, not decimals: 13.5 stayed 13.5, and 3.14159 printed 3.14. After std::fixed it counts digits after the point, and both settings stay on: price printed 4.50 with nothing new sent. std::left puts the padding after the text. std::defaultfloat turns std::fixed off again, but the precision stays: 13.5 then printed 14 until std::setprecision(6), the default, brought back 13.5.

How do you print a bool as true or false?

Send std::boolalpha first: by default a bool prints as 1 or 0.

bool in_stock = true;
bool on_sale = false;

std::cout << in_stock << ' ' << on_sale << '\n';
std::cout << std::boolalpha;  // from here on: true and false
std::cout << in_stock << ' ' << on_sale << '\n';

The first line printed 1 0 and the second true false. std::noboolalpha switches back to digits.

std::cout vs printf vs std::println: which one?

In C++17 code, use std::cout; from C++23 on, std::println gives you a short format string and still checks the types. The same line three ways:

std::cout << count << " x " << item << ": " << std::fixed
          << std::setprecision(2) << total << '\n';
std::printf("%d x %s: %.2f\n", count, item.c_str(), total);
std::println("{} x {}: {:.2f}", count, item, total);  // C++23

Built as C++23, all three printed 3 x coffee: 13.50. As C++17 the std::println line does not compile, and it needs #include <print>. std::printf needs <cstdio>, and it takes a std::string as item.c_str(), the C string inside it.

std::println in C++23 shows its format strings, and Since When lists the compiler releases that have it. Reading input, the other direction, is std::cin and C++ getline; Standard library has the rest of the library’s answers.