This rule is part of MISRA C++:2023.
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Rule 11.6.1 - All variables should be initialized
[dcl.init]
Category: Advisory
Analysis: Decidable,Single Translation Unit
Amplification
All variables should either be explicitly or implicitly initialized.
Apart from the following, all variables should be explicitly initialized with an associated initializer in their definition:
- Variables of
class type, or
- Function parameters (which are initialized with the corresponding argument value), or
- Variables with static storage duration (which are zero-initialized by default).
Rationale
Having several states within a program increases the risk of defects being introduced. Each variable that is first uninitialized, then set to a
value creates two program states. It is therefore better to initialize the variable directly to a value that is to be used. The intent of this rule is
not that each variable is initialized with some value, but that it is initialized with its real value; the one that will be used when the variable is
next read.
In order to achieve this, the variable definition can be delayed until the "right" value is available. This naturally leads to reducing the
variable’s scope, reducing the risk of the variable being used inappropriately. An immediately evaluated lambda can be used to compute a value when a
variable’s initialization is more complex.
In many cases, initializing the variable within its definition allows it to be a constant definition.
Note: there are many ways to explicitly initialize a variable. When possible, the list-initialization syntax (with curly braces)
should be used as it does not suffer from the issues that arise from the use of other syntactic forms (e.g. narrowing or declaring a function while
trying to define a variable, also known as "the most vexing parse").
Example
void f( bool cond )
{
int32_t i; // Non-compliant
if ( cond ) { i = 42; }
else { i = -1; }
int32_t j = cond ? 42 : -1; // Compliant
int32_t k = [&]() // Compliant
{
if ( cond ) { return 42; }
else { return -1; }
}();
string s; // Compliant - default-initialized
}
int32_t g; // Compliant - static initialization applies
void f()
{
thread_local int32_t i; // Compliant - static initialization applies
}
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