C++ Format Specifiers

In this page:

Floating-Point Formatting
Hexadecimal and Octal Formats
Boolean Output Format
Scientific Notation
Base Prefix Formatting

Floating-Point Formatting

The fixed stream manipulator forces floating-point numbers to print in standard decimal notation rather than switching to scientific notation for very large or small values. Combine it with setprecision to control exactly how many digits appear after the decimal point, which is essential for anything like displaying money.

Example: Floating-Point Formatting

cpp

#include <iostream>
#include <iomanip>

int main() {
	double price = 19.5;
	std::cout << std::fixed << std::setprecision(2) << price << std::endl;
	return 0;
}

Hexadecimal and Octal Formats

The hex and oct manipulators switch how subsequent integers print — hex shows them in base-16 and oct in base-8 — while dec switches back to the normal base-10 you'd expect. These are especially useful when debugging low-level code, where seeing a value's hexadecimal form makes bit patterns and memory addresses much easier to read.

Example: Hexadecimal and Octal Formats

cpp

#include <iostream>

int main() {
	int value = 255;
	std::cout << std::hex << value << std::endl;
	std::cout << std::oct << value << std::endl;
	std::cout << std::dec << value << std::endl;
	return 0;
}

Boolean Output Format

By default C++ prints boolean values as the raw integers 1 and 0, which can be confusing in output meant for a human reader. Applying boolalpha makes cout print the actual words true and false instead, and noboolalpha switches back to the numeric form if you need it again later.

Example: Boolean Output Format

cpp

#include <iostream>

int main() {
	bool active = true;
	std::cout << active << std::endl; // 1
	std::cout << std::boolalpha << active << std::endl; // true
	return 0;
}

Scientific Notation

scientific formats floating-point numbers using exponential notation, like 1.5e+08 instead of 150000000, which keeps very large or very small numbers compact and easy to compare at a glance. This matters in scientific or engineering code where values can span many orders of magnitude in the same output.

Example: Scientific Notation

cpp

#include <iostream>

int main() {
	double bigNumber = 150000000.0;
	std::cout << std::scientific << bigNumber << std::endl;
	return 0;
}

Base Prefix Formatting

showbase adds a visible prefix that identifies a number's base — 0x for hexadecimal and a leading 0 for octal — so a reader doesn't have to guess how to interpret the digits that follow. It's most useful alongside hex or oct, since without it a hex value like ff could easily be mistaken for decimal.

Example: Base Prefix Formatting

cpp

#include <iostream>

int main() {
	int value = 255;
	std::cout << std::showbase << std::hex << value << std::endl;
	return 0;
}

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