A4-7-1 - Unofficial Autosar C++ Guidelines Reference

Rule A4-7-1 (required, implementation, automated)

An integer expression shall not lead to data loss.

Rationale

Implicit conversions, casts and arithmetic expressions may lead to data loss, e.g. overflows, underflows or wrap-around. Integral expressions need to be performed on proper integral types that ensure that the data loss will not occur or appropriate guards should be used to statically detect or counteract such a data loss.

Example

// $Id: A4-7-1.cpp 289436 2017-10-04 10:45:23Z michal.szczepankiewicz $
#include <cstdint>
#include <stdexcept>
std::int8_t Fn1(std::int8_t x, std::int8_t y) noexcept
{
return (x + y); // Non-compliant - may lead to overflow

}
std::int8_t Fn2(std::int8_t x, std::int8_t y)
{
if (x > 100 || y > 100) // Range check
{
throw std::logic_error("Preconditions check error");
}
return (x + y); // Compliant - ranges of x and y checked before the
// arithmetic operation
}
std::int16_t Fn3(std::int8_t x, std::int8_t y) noexcept
{
return (static_cast<std::int16_t>(x) + y); // Compliant - std::int16_t type
// is enough
// operation
}
std::uint8_t Fn4(std::uint8_t x, std::uint8_t y) noexcept
{

for this arithmetic

return (x * y); // Non-compliant - may lead to wrap-around

}
std::int8_t Fn5(std::int16_t x)
{
return static_cast<std::int8_t>(x); // Non-compliant - data loss
}
std::int8_t Fn6(std::int16_t x)
{
return x; // Non-compliant - data loss by implicit conversion
}
void F()
{
std::int8_t x1 =
Fn1(5, 10); // Compliant - overflow will not occur for these values
std::int8_t x2 = Fn1(250, 250); // Non-compliant - Overflow occurs
try
{
std::int8_t x3 =
Fn2(250, 250); // Compliant - No overflow, range checks
// inside fn2() function
}
catch (std::logic_error&)
{
// Handle an error
}
std::int16_t x4 = Fn3(250, 250); // Compliant - No overflow, arithmetic
// operation underlying type is wider than
// std::int8_t
std::uint8_t x5 = Fn4(50, 10);// Non-compliant - Wrap-around occurs
std::int8_t x6 = Fn5(100);// Compliant - data loss will not occur
std::int8_t x7 = Fn5(300);// Non-compliant - Data loss occurs
std::int8_t x8 = Fn6(300);// Non-compliant - Data loss occurs

std::int8_t x9 = 150;
std::int16_t x10 = static_cast<std::int16_t>(x9 + x9); // Non-compliant
x10 = x9 + x9;
x10 = static_cast<std::int16_t>(x9) + x9;

// Non-compliant
// Compliant

std::int8_t x11 = x9 << 5; // Non-compliant

std::int8_t x12 = 127;
++x12; // Non-compliant

std::uint8_t x13 = 255;
++x13; // Non-compliant

}

Unofficial Autosar C++ Guidelines Reference

Rationale

Example

See also