Question

Is the following code really dangerous? doSmth returns object of vector, which should be copied from cont(doSmth) and then stored in a stack in the scope of function test. Thus, I believe that it will be desctructed only after returning from test.

struct MyData
{
  double m_i;
};

std::vector<MyData> doSmth()
{
  std::vector<MyData> cont(10);
  return cont;
}

void test()
{
  MyData& oneElement = doSmth()[0];
  std::cout << oneElement.m_i << std::endl;
}

Yet valgrind thinks differently:

Invalid read of size 8
<line 1 in test function oneElement>
Address 0x101281db8 is 8 bytes inside a block of size 72 free'd
std::vector<BlockInfo, std::allocator<BlockInfo> >::~vector() (stl_vector.h:314)
<line 2 in test function>

Is it a problem in my logic or valgrind lies?

Answer 1

The return value is destroyed at the end of the full expression, i.e. at the first semicolon in the function test. At that point, oneElement becomes a dangling reference.

You can prolong the lifetime of the return value by binding it to a const reference:

void test()
{
    std::vector<MyData> const & r = doSmth();
    MyData const & oneElement = r[0];
    std::cout << oneElement.m_i << std::endl;
}

In the above case, the return value is destroyed at the end of the function.