C++ Type Conversion

Type Conversion

Sometimes you need to change a value from one data type to another, like a decimal number to a whole number. This is called type conversion.

There are two kinds:

• Automatic conversion - C++ does it for you
• Manual conversion - you tell C++ to do it

Automatic Conversion

When you store a value of one type in a variable of another type, C++ converts it automatically.

An int can be stored in a double without any problem:

int myInt = 9;
double myDouble = myInt;  // int to double

cout << myDouble;

But when you store a double in an int, the decimals are lost:

double price = 9.99;
int wholePrice = price;  // double to int

cout << wholePrice;

Manual Conversion

You can also convert a value yourself. In C++, the recommended way is static_cast. Write the new type inside < >, and the value inside the parentheses:

double myDouble = 9.78;
int myInt = static_cast<int>(myDouble);

cout << myDouble << "\n";
cout << myInt;

You may also see the older C-style way in other code, where you write the new type in parentheses in front of the value:

double myDouble = 9.78;
int myInt = (int) myDouble;

cout << myInt;

It does the same thing, but static_cast is easier to spot and safer to use.

Why Is Conversion Important?

When you divide two int values, the result is also an int, and the decimals are lost. This often gives the wrong answer, for example when you calculate an average:

int sum = 7;
int count = 2;

double wrong = sum / count;
double right = static_cast<double>(sum) / count;

cout << wrong << "\n";
cout << right;

7 / 2 is calculated with whole numbers first, so the result is 3, even though it is stored in a double. Converting one of the values to double before dividing gives the correct answer, 3.5.

Real-Life Example

Here is a real-life example of type conversion. A game gives the player a score out of 500. The program shows the score as a percentage, and as stars (one star for every 20 percent):

int maxScore = 500;
int userScore = 423;

// Convert to double, so the division keeps the decimals
double percentage = static_cast<double>(userScore) / maxScore * 100;

// Convert to int, to get whole stars (one star for every 20 percent)
int stars = static_cast<int>(percentage / 20);

cout << "Score: " << percentage << "%\n";
cout << "Stars: " << stars;

userScore is converted to double before dividing, so the decimals are kept (84.6). Then percentage / 20 (4.23) is converted to int, which cuts off the decimals and gives 4 stars.