C++ auto auto and proxy objects


Sometimes auto may behave not quite as was expected by a programmer. It type deduces the expression, even when type deduction is not the right thing to do.

As an example, when proxy objects are used in the code:

std::vector<bool> flags{true, true, false};
auto flag = flags[0];

Here flag would be not bool, but std::vector<bool>::reference, since for bool specialization of template vector the operator [] returns a proxy object with conversion operator operator bool defined.

When flags.push_back(true) modifies the container, this pseudo-reference could end up dangling, referring to an element that no longer exists.

It also makes the next situation possible:

void foo(bool b);

std::vector<bool> getFlags();

auto flag = getFlags()[5];

The vector is discarded immediately, so flag is a pseudo-reference to an element that has been discarded. The call to foo invokes undefined behavior.

In cases like this you can declare a variable with auto and initialize it by casting to the type you want to be deduced:

auto flag = static_cast<bool>(getFlags()[5]);

but at that point, simply replacing auto with bool makes more sense.

Another case where proxy objects can cause problems is expression templates. In that case, the templates are sometimes not designed to last beyond the current full-expression for efficiency sake, and using the proxy object on the next causes undefined behavior.