MISRA C++:2008 Rule 5-0-8

An explicit integral or floating-point conversion shall not increase the size of the underlying type of a cvalue expression

Description

Rule Definition

An explicit integral or floating-point conversion shall not increase the size of the underlying type of a cvalue expression.

Rationale

Expressions flagged by this checker follow the detailed specifications for cvalue expressions from the MISRA^® C++ documentation.

If you evaluate an expression and later cast the result to a different type, the cast has no effect on the underlying type of the evaluation (the widest of operand data types in the expression). For instance, in this example, the sum of two short operands is cast to the wider type int.

short op1;
short op2;
int res;
res= static_cast<int> (op1 + op2);

However, a developer or code reviewer can expect that the evaluation uses the data type to which the result is cast later. For instance, one can expect a sum with the underlying type int because of the later cast.

Additional Message in Report

An explicit integral or floating-point conversion shall not increase the size of the underlying type of a cvalue expression.

Complex expression of underlying type typeBeforeConversion may only be cast to narrower integer type of same signedness, however the destination type is typeAfterconversion.

Troubleshooting

If you expect a rule violation but do not see it, refer to Coding Standard Violations Not Displayed.

Examples

Conversion of Sum to Wider Integer Type

void func() {
     short op1;
     short op2;
     int res;

     res = static_cast<int> (op1 + op2); //Noncompliant
     res = static_cast<int> (op1) + op2; //Compliant

}

In this example, the first cast on the sum violates the rule but the second cast does not.

• The first cast can lead to the incorrect expectation that the sum is evaluated with an underlying type int.

• The second cast first converts one of the operands to int so that the sum is actually evaluated with the underlying type int.

Check Information

Group: Expressions

Category: Required