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4.4 — Implicit type conversion (coercion)

Previously, you learned that a value of a variable is stored as a sequence of bits, and the data type of the variable tells the compiler how to interpret those bits into meaningful values. Different data types may represent the “same” number differently -- for example, the integer value 3 and the float value 3.0 are stored as completely different binary patterns.

So what happens when we do something like this?

In such a case, the compiler can’t just copy the bits representing the value 3 into float f. Instead, it needs to convert the integer 3 to a floating point number, which can then be assigned to variable f.

The process of converting a value from one data type to another is called a type conversion. Type conversions can happen in many different cases:

Assigning to or initializing a variable with a value of a different data type:

Passing a value to a function where the function parameter is of a different data type:

Returning a value from a function where the function return type is of a different data type:

Using a binary operator with operands of different types:

In all of these cases (and quite a few others), C++ will use type conversion to convert data from one type to another.

There are two basic types of type conversion: implicit type conversion, where the compiler automatically transforms one fundamental data type into another, and explicit type conversions, where the developer uses a casting operator to direct the conversion.

We’ll cover implicit type conversion in this lesson, and explicit type conversion in the next.

Implicit type conversion

Implicit type conversion (also called automatic type conversion or coercion) is performed whenever one fundamental data type is expected, but a different fundamental data type is supplied, and the user does not explicitly tell the compiler how to perform this conversion (via a cast).

All of the above examples are cases where implicit type conversion will be used.

There are two basic types of implicit type conversion: promotions and conversions.

Numeric promotion

Whenever a value from one type is converted into a value of a larger similar data type, this is called a numeric promotion (or widening, though this term is usually reserved for integers). For example, an int can be widened into a long, or a float promoted into a double:

While the term “numeric promotion” covers any type of promotion, there are two other terms with specific meanings in C++:

Integral promotion and floating point promotion are used in specific cases to convert smaller data types to int/unsigned int or double, because those data types are generally the most performant to perform operations on.

The important thing to remember about promotions is that they are always safe, and no data loss will result.

Numeric conversions

When we convert a value from a larger type to a similar smaller type, or between different types, this is called a numeric conversion. For example:

Unlike promotions, which are always safe, conversions may or may not result in a loss of data. Because of this, any code that does an implicit conversion will often cause the compiler to issue a warning.

The rules for conversions are complicated and numerous, so we’ll just cover the common cases here.

In all cases, converting a value into a type that doesn’t have a large enough range to support the value will lead to unexpected results. This should be avoided. For example:

In this example, we’ve assigned a large integer to a char (that has range -128 to 127). This causes the char to overflow, and produces the result:

48

However, converting from a larger integral or floating point type to a smaller similar type will generally work so long as the value fits in the range of the smaller type. For example:

This produces the expected result:

2
0.1234

In the case of floating point values, some rounding may occur due to a loss of precision in the smaller type. For example:

In this case, we see a loss of precision because the float can’t hold as much precision as a double:

0.123456791

Converting from an integer to a floating point number generally works as long as the value fits. For example:

This produces the expected result:

10

Converting from a floating point to an integer works as long as the value fits, but any fractional values are lost. For example:

In this example, the fractional value (.5) is lost, leaving the following result:

3

Evaluating arithmetic expressions

When evaluating expressions, the compiler breaks each expression down into individual subexpressions. The arithmetic operators require their operands to be of the same type. To ensure this, the compiler uses the following rules:

The priority of operands is as follows:

We can see the usual arithmetic conversion take place via use of the typeid() operator (included in the typeinfo header), which can be used to show the resulting type of an expression.

In the following example, we add two shorts:

Because shorts are integers, they undergo integral promotion to ints before being added. The result of adding two ints is an int, as you would expect:

int 9

Note: Your compiler may display something slightly different as the format of typeid.name() is left up to the compiler.

Let’s take a look at another case:

In this case, the short undergoes integral promotion to an int. However, the int and double still do not match. Since double is higher on the hierarchy of types, the integer 2 gets converted to the double 2.0, and the doubles are added to produce a double result.

double 6.0

This hierarchy can cause some interesting issues. For example, take a look at the following code:

you might expect the expression 5u - 10 to evaluate to -5 since 5 - 10 = -5. But here’s what actually happens:

4294967291

In this case, the signed integer (10) is promoted to an unsigned integer (which has higher priority), and the expression is evaluated as an unsigned int. Overflow results, and we get an answer we don’t expect.

This is one of many good reasons to avoid unsigned integers in general.

Warning: Microsoft’s Visual C++ 2005 does not seem to issue warnings for unsafe signed/unsigned conversions.

4.4a -- Explicit type conversion (casting) [1]
Index [2]
4.3c -- Using statements [3]