4.12 — Introduction to type conversion and static_cast

Implicit type conversion

Consider the following program:

#include <iostream>

void print(double x) // print takes a double parameter
{
	std::cout << x << '\n';
}

int main()
{
	print(5); // what happens when we pass an int value?

	return 0;
}

In the above example, the print() function has a parameter of type double but the caller is passing in the value 5 which is of type int. What happens in this case?

In most cases, C++ will allow us to convert values of one fundamental type to another fundamental type. The process of converting a value from one type to another type is called type conversion. Thus, the int argument 5 will be converted to double value 5.0 and then copied into parameter x. The print() function will print this value, resulting in the following output:

5

When the compiler does type conversion on our behalf without us explicitly asking, we call this implicit type conversion. The above example illustrates this -- nowhere do we explicitly tell the compiler to convert integer value 5 to double value 5.0. Rather, the function is expecting a double value, and we pass in an integer argument. The compiler will notice the mismatch and implicitly convert the integer to a double.

Here’s a similar example where our argument is an int variable instead of an int literal:

#include <iostream>

void print(double x) // print takes a double parameter
{
	std::cout << x << '\n';
}

int main()
{
	int y { 5 };
	print(y); // y is of type int

	return 0;
}

This works identically to the above. The value held by int variable y (5) will be converted to double value 5.0, and then copied into parameter x.

Type conversion produces a new value

Even though it is called a conversion, a type conversion does not actually change the value or type of the value being converted. Instead, the value to be converted is used as input, and the conversion results in a new value of the target type (via direct initialization).

In the above example, the conversion does not change variable y from type int to double. Instead, the conversion uses the value of y (5) as input to create a new double value (5.0). This double value is then passed to function print.

Implicit type conversion warnings

Although implicit type conversion is sufficient for most cases where type conversion is needed, there are a few cases where it is not. Consider the following program, which is similar to the example above:

#include <iostream>

void print(int x) // print now takes an int parameter
{
	std::cout << x << '\n';
}

int main()
{
	print(5.5); // warning: we're passing in a double value

	return 0;
}

In this program, we’ve changed print() to take an int parameter, and the function call to print() is now passing in double value 5.5. Similar to the above, the compiler will use implicit type conversion in order to convert double value 5.5 into a value of type int, so that it can be passed to function print().

Unlike the initial example, when this program is compiled, your compiler will generate some kind of a warning about a possible loss of data. And because you have “treat warnings as errors” turned on (you do, right?), your compiler will abort the compilation process.

When compiled and run, this program prints the following:

5

Note that although we passed in value 5.5, the program printed 5. Because integral values can’t hold fractions, when double value 5.5 is implicitly converted to an int, the fractional component is dropped, and only the integral value is retained.

Because converting a floating point value to an integral value results in any fractional component being dropped, the compiler will warn us when it does an implicit type conversion from a floating point to an integral value. This happens even if we were to pass in a floating point value with no fractional component, like 5.0 -- no actual loss of value occurs during the conversion to integral value 5 in this specific case, but the compiler may still warn us that the conversion is unsafe.

int main()
{
    double d { 5 }; // okay: int to double is safe
    int x { 5.5 }; // error: double to int not safe

    return 0;
}

An introduction to explicit type conversion via the static_cast operator

Back to our most recent print() example, what if we intentionally wanted to pass a double value to a function taking an integer (knowing that the converted value would drop any fractional component?) Turning off “treat warnings as errors” just to make our program compile is a bad idea, because then we’ll have warnings every time we compile (which we will quickly learn to ignore), and we risk overlooking warnings about more serious issues.

C++ supports a second method of type conversion, called explicit type conversion. Explicit type conversion allow us (the programmer) to explicitly tell the compiler to convert a value from one type to another type, and that we take full responsibility for the result of that conversion (meaning that if the conversion results in the loss of value, it’s our fault).

To perform an explicit type conversion, in most cases we’ll use the static_cast operator. The syntax for the static cast looks a little funny:

static_cast<new_type>(expression)

static_cast takes the value from an expression as input, and returns that value converted into the type specified by new_type (e.g. int, bool, char, double).

Let’s update our prior program using static_cast:

#include <iostream>

void print(int x)
{
	std::cout << x << '\n';
}

int main()
{
	print( static_cast<int>(5.5) ); // explicitly convert double value 5.5 to an int

	return 0;
}

Because we’re now explicitly requesting that double value 5.5 be converted to an int value, the compiler will not generate a warning about a possible loss of data upon compilation (meaning we can leave “treat warnings as errors” enabled).

Using static_cast to convert char to int

In the lesson on chars 4.11 -- Chars, we saw that printing a char value using std::cout results in the value being printed as a char:

#include <iostream>

int main()
{
    char ch{ 97 }; // 97 is ASCII code for 'a'
    std::cout << ch << '\n';

    return 0;
}

This prints:

a

If we want to print the integral value instead of the char, we can do this by using static_cast to cast the value from a char to an int:

#include <iostream>

int main()
{
    char ch{ 97 }; // 97 is ASCII code for 'a'
    std::cout << ch << " has value " << static_cast<int>(ch) << '\n'; // print value of variable ch as an int

    return 0;
}

This prints:

a has value 97

It’s worth noting that the argument to static_cast evaluates as an expression. When we pass in a variable, that variable is evaluated to produce its value, and that value is then converted to the new type. The variable itself is not affected by casting its value to a new type. In the above case, variable ch is still a char, and still holds the same value even after we’ve cast its value to an int.

Converting unsigned numbers to signed numbers

To convert an unsigned number to a signed number, you can also use the static_cast operator:

#include <iostream>

int main()
{
    unsigned int u { 5 };
    int s { static_cast<int>(u) }; // return value of variable u as an int

    std::cout << s << '\n';
    return 0;
}

The static_cast operator doesn’t do any range checking, so if you cast a value to a type whose range doesn’t contain that value, undefined behavior will result. Therefore, the above cast from unsigned int to int will yield unpredictable results if the value of the unsigned int is greater than the maximum value a signed int can hold.

std::int8_t and std::uint8_t likely behave like chars instead of integers

As noted in lesson 4.6 -- Fixed-width integers and size_t, most compilers define and treat std::int8_t and std::uint8_t (and the corresponding fast and least fixed-width types) identically to types signed char and unsigned char respectively. Now that we’ve covered what chars are, we can demonstrate where this can be problematic:

#include <cstdint>
#include <iostream>

int main()
{
    std::int8_t myInt{65};      // initialize myInt with value 65
    std::cout << myInt << '\n'; // you're probably expecting this to print 65

    return 0;
}

Because std::int8_t describes itself as an int, you might be tricked into believing that the above program will print the integral value 65. However, on most systems, this program will print A instead (treating myInt as a signed char). However, this is not guaranteed (on some systems, it may actually print 65).

If you want to ensure that a std::int8_t or std::uint8_t object is treated as an integer, you can convert the value to an integer using static_cast:

#include <cstdint>
#include <iostream>

int main()
{
    std::int8_t myInt{65};
    std::cout << static_cast<int>(myInt) << '\n'; // will always print 65

    return 0;
}

In cases where std::int8_t is treated as a char, input from the console can also cause problems:

#include <cstdint>
#include <iostream>

int main()
{
    std::cout << "Enter a number between 0 and 127: ";
    std::int8_t myInt{};
    std::cin >> myInt;

    std::cout << "You entered: " << static_cast<int>(myInt) << '\n';

    return 0;
}

A sample run of this program:

Enter a number between 0 and 127: 35
You entered: 51

Here’s what’s happening. When std::int8_t is treated as a char, the input routines interpret our input as a sequence of characters, not as an integer. So when we enter 35, we’re actually entering two chars, '3' and '5'. Because a char object can only hold one character, the '3' is extracted (the '5' is left in the input stream for possible extraction later). Because the char '3' has ASCII code point 51, the value 51 is stored in myInt, which we then print later as an int.

In contrast, the other fixed-width types will always print and input as integral values.

Quiz time

Question #1

Write a short program where the user is asked to enter a single character. Print the value of the character and its ASCII code, using static_cast.

The program’s output should match the following:

Enter a single character: a
You entered 'a', which has ASCII code 97.

Show Solution

#include <iostream>

int main()
{
	std::cout << "Enter a single character: ";
	char c{};
	std::cin >> c;
	std::cout << "You entered '" << c << "', which has ASCII code " << static_cast<int>(c) << ".\n";

	return 0;
}

Question #2

Modify the program you wrote for quiz #1 to use implicit type conversion instead of static_cast. How many different ways can you think of to do this?

Note: You should favor explicit conversions over implicit conversions, so don’t actually do this in real programs -- this is just to test your understanding of where implicit conversions can occur.

Show Solution

#include <iostream>

int main()
{
	std::cout << "Enter a single character: ";
	char c{};
	std::cin >> c;

	int ascii{ c };
	std::cout << "You entered '" << c << "', which has ASCII code " << ascii << ".\n";

	return 0;
}

#include <iostream>

int charAsInt(char c)
{
	return c;
}

int main()
{
	std::cout << "Enter a single character: ";
	char c{};
	std::cin >> c;

	std::cout << "You entered '" << c << "', which has ASCII code " << charAsInt(c) << ".\n";

	return 0;
}

#include <iostream>

int getInt(int c)
{
	return c;
}

int main()
{
	std::cout << "Enter a single character: ";
	char c{};
	std::cin >> c;

	std::cout << "You entered '" << c << "', which has ASCII code " << getInt(c) << ".\n";

	return 0;
}