Common data types

Overview

Teaching: 30 min
Exercises: 15 min

Questions

• What are the common, built-in data types in C++pointers and references in C++

• When should I use each data type?

Objectives

• Learn about the built-in data types in C++, and what they are used for

Lesson Contents

• Prerequisites
• sizeof operator
• Exercise
• Solution
• Integral data types
• Exercise
• Solution
• Floating point data types
• Variable initialization
• Exercise
• Solution
• Compiler warnings
• Exercise

Prerequisites

• Understanding of basic C++ syntax
• Able to compile and run simple C++ programs

sizeof operator

C/C++ has an operator sizeof that can apply to a data type or variable. This returns the size of the object in bytes. You can use this to see how much memory a particular variable or data type holds.

#include <iostream>

int main(void)
{
    double number = 1.234;
    std::cout << "int: " << sizeof(int) << std::endl;
    std::cout << "double: " << sizeof(number) << std::endl;
    return 0;
}

int: 4
double: 8

The output is technically dependent on your operating system and architecture, although 4-byte int is very common, and 8-byte (64-bit) double is farily universal.

Exercise

What is the size of a bool?

Solution

A bool is usually 1 bytes (8 bits), even though it only really needs 1 bit. This is because A byte is generally the smallest quantity a program works with.

Integral data types

C++ has several integer data types

These types can hold both positive and negative values. We can modify this behavior, though, with the unsigned keyword. This increases the range on the positive side, at the expense of not being able to hold negative values.

unsigned char is often used to represent a raw byte.

In general, signed data types are preferred over unsigned, and int is typically used as the “general purpose” integer number, with the others being used in specialized cases.

Exercise

What happens if you assign a negative value to an unsigned int?

Solution

The value of the unsigned int becomes a large positive number. The negative number wraps around to become positive.

Floating point data types

C++ has a couple of floating-point (decimal) types as well.

There are no unsigned floating point types.

long double may or may not really have 16-bytes (128-bits) of precision, and is rarely used. CPUs generally understand 32-bit floats and 64-bit doubles much better, and these are very efficient.

Variable initialization

It is generally good practice to initialize variables to a set value. In general, the value of a variable is not set by the compiler, and may be any value that happens to be in the memory that that variable has.

Exercise

Print an uninitialized double. What is the value? Does that value change each time that program runs?

Solution

#include <iostream>

int main(void)
{
   double d;
   std::cout << d << std::endl;
   return 0;
}

The value printed is random, and changes each time you run the program.

Compiler warnings

Compilers have settings to warn you of dangerous behavior like the use of unitialized variables. We can enable these warnings with the GCC and Clang compilers with -Wall.

Exercise

Compile the code from the previous exercise (with the unitialized variable) using -Wall. What does the compiler output?

It is generally good practice to compile with -Wall (or other flags, depending on the compiler) such that common mistakes (but not necessarily errors) are caught by the compiler.

Key Points

• C++ has several integral data types, with different sizes and use cases

• C++ also has several different floating point types