AUTOSAR C++14 Rule A12-8-3

Moved-from object shall not be read-accessed

Since R2021a

Description

Rule Definition

Moved-from object shall not be read-accessed.

Rationale

Because the content of a source object is generally unspecified after a move operation, it is unsafe to perform operations that access the contents of the source object after a move operation. Accessing the contents of the source object after a move operation might result in a data integrity violation, an unexpected value, or an illegal dereferencing of a pointer.

Operations that make no assumptions about the state of an object do not violate this rule.

The C++ standard specifies that these move operations leave the source object in a well-specified state after the move:

• Move construction, move assignment, converting¹ move construction, and converting move assignment of std::unique_ptr type

• Move construction, move assignment, converting move construction, converting move assignment of std::shared_ptr type

• Move construction and move assignment from a std::unique_ptr of std::shared_ptr type

• Move construction, move assignment, converting move construction, and converting move assignment of std::weak_ptr type

• std::move() of std::basic_ios type

• Move constructor and move assignment of std::basic_filebuf type

• Move constructor and move assignment of std::thread type

• Move constructor and move assignment of std: unique_lock type

• Move constructor and move assignment of std::shared_lock type

• Move constructor and move assignment of std::promise type

• Move constructor and move assignment of std::future type

• Move construction, move assignment, converting move construction, and converting move assignment of std::shared_future type

• Move constructor and move assignment of std::packaged_task type

Because these move operations leave the source object in a well-specified state, accessing the source object after calling these functions is compliant with this rule.

Polyspace Implementation

Polyspace^® raises a flag if the source object is read after its contents are moved to a destination object by calling the std::move function explicitly. Polyspace does not flag accessing a source object if:

• The source object of an explicit move operation is of these types:

  • std::unique_ptr

  • std::shared_ptr

  • std::weak_ptr

  • std::basic_ios

  • std::basic_filebuf

  • std::thread

  • std::unique_lock

  • std::shared_lock

  • std::promise

  • std::future

  • std::shared_future

  • std::packaged_task

• The move operation is performed implicitly. For instance, the function std::remove moves objects implicitly. Polyspace does not flag accessing the object moved implicitly. To avoid accidentally accessing a moved object, erase the removed object using std::erase. For details about using std::remove, see Improper erase-remove idiom.

• The source object is of a built-in base type, such as: int, enum, float, double, pointer, std::intptr_t, std::nullptr_t.

Troubleshooting

If you expect a rule violation but Polyspace does not report it, see Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

Avoid Reading Source Object After Calling std::move

This example shows how Polyspace flags reading the source object after an explicit move operation.

#include<string>
#include<iostream>
void F1()
{
	std::string s1{"string"};
	std::string s2{std::move(s1)}; 
	// ...
	std::cout
	<<  // Noncompliant
	s1
	<< "\n";
}

void F2()
{
	std::unique_ptr<std::int32_t> ptr1 = std::make_unique<std::int32_t>(0);
	std::unique_ptr<std::int32_t> ptr2{std::move(ptr1)};
	std::cout << ptr1.get() << std::endl; // Compliant by exception
}
void g(std::string v)
{
	std::cout << v << std::endl; 
}

void F3()
{
	std::string s;
	for (unsigned i = 0; i < 10; ++i) {
		s.append(1, static_cast<char>('0' + i));  //Noncompliant 
		g(std::move(s));
	}
}
void F4()
{
	for (unsigned i = 0; i < 10; ++i) {
		std::string s(1, static_cast<char>('0' + i)); // Compliant
		g(std::move(s));  
	}
}

• In the function F1, string s1 is explicitly moved to s2 by calling std::move. After the move operation, the function attempts to read s1. Polyspace flags this attempt of reading a source object after an explicit move.

• In the function F2, the unique pointer ptr1 is explicitly moved to ptr2. Because the std::unique_ptr remains in a specified state after the move, reading a source unique pointer after an explicit move is compliant with this rule.

• In the function F3, the string s is explicitly moved, and then it is read by the std::string::append function. Polyspace flags this attempt of reading a source object after an explicit move.

• In the function F4, the string s is explicitly moved. In each iteration of the loop, s is initiated to specific content before the move operation is triggered. As a result, the state of s is specified before the object is accessed. This method of accessing the source object after a move operation is compliant with this rule.

Check Information

Group: Special member functions

Category: Required, Partially automated

Version History

Introduced in R2021a