Perform at most one explicit resource allocation in a single expression statement

Reason

If you perform two explicit resource allocations in one statement, you could leak resources because the order of evaluation of many subexpressions, including function arguments, is unspecified.

Example

void fun(shared_ptr<Widget> sp1, shared_ptr<Widget> sp2);

This fun can be called like this:

// BAD: potential leak
fun(shared_ptr<Widget>(new Widget(a, b)), shared_ptr<Widget>(new Widget(c, d)));

This is exception-unsafe because the compiler might reorder the two expressions building the function's two arguments. In particular, the compiler can interleave execution of the two expressions: Memory allocation (by calling operator new) could be done first for both objects, followed by attempts to call the two Widget constructors. If one of the constructor calls throws an exception, then the other object's memory will never be released!

This subtle problem has a simple solution: Never perform more than one explicit resource allocation in a single expression statement. For example:

shared_ptr<Widget> sp1(new Widget(a, b)); // Better, but messy
fun(sp1, new Widget(c, d));

The best solution is to avoid explicit allocation entirely, use factory functions that return owning objects:

fun(make_shared<Widget>(a, b), make_shared<Widget>(c, d)); // Best

Write your own factory wrapper if there is not one already.

Enforcement

• Flag expressions with multiple explicit resource allocations (problem: how many direct resource allocations can we recognize?)