This rule is part of MISRA C++:2023.
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Rule 11.6.2 - The value of an object must not be read before it has been set
[dcl.init] Undefined 12
Category: Mandatory
Analysis: Undecidable,System
Amplification
For the purposes of this rule, an array element or class member is considered to be a discrete object.
Note: struct members are also class members.
Rationale
Objects created with automatic storage duration or dynamic storage duration have an indeterminate value. Reading an
indeterminate value may result in undefined behaviour.
This rule requires that all objects are written, either by implicit or explicit initialization in their declaration or by assignment, before they
are read.
Note: jumping over an initializer by the use of a goto or switch statement "bypasses" the declaration of
the object, rendering the program ill-formed.
Example
namespace
{
int32_t Z; // Compliant - implicitly initialized with '0'
}
void f()
{
int32_t i;
int32_t j = i + 1; // Non-compliant - i has not been assigned a value
int32_t * p = new int32_t;
int32_t k = *p; // Non-compliant - *p has not been assigned a value
int32_t * q;
if ( q == p ) // Non-compliant - q has not been assigned a value
{
}
}
int32_t g( bool b )
{
if ( b )
{
goto L1;
}
int32_t x;
x = 10u;
L1:
x = x + 1u; // Non-compliant - x may not have been assigned a value
return x;
}
struct S { int32_t a; int32_t b; };
void h()
{
S s1;
S s2 = { 10 };
auto i1 = s1.a; // Non-compliant
auto i2 = s2.b; // Compliant - s2.b implicitly initialized to 0
int32_t array1[ 10 ] = { 1, 2, 3 };
int32_t array2[ 10 ];
auto i3 = array1[ 5 ]; // Compliant - array1[ 5 ] implicitly initialized to 0
auto i4 = array2[ 5 ]; // Non-compliant
}
class C
{
public:
C() : m_a( 10 ), m_b( 7 ) // Both m_a and m_b initialized
{
}
C( int32_t a ) : m_a( a ) // m_b not initialized
{
}
int32_t GetmB()
{
return m_b;
}
private:
int32_t m_a;
int32_t m_b;
};
int main()
{
C c1;
if ( c1.GetmB() > 0 ) // Compliant - m_b initialized
{
}
C c2( 5 );
if ( c2.GetmB() > 0 ) // Non-compliant - m_b not initialized
{
}
}
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