Order of execution in constructor initialization list

Is order of execution in constructor initialization list determinable? I know that members order in a class is the order in which those members will be initialized but if I have scenario like this:

class X()
{
X_Implementation* impl_;
};  

and then providing that allocator is available:

X::X():impl_(Allocate(sizeof(X_Implementation)))//HERE I'M ALLOCATING <--1
,impl_(Construct<X_Implementation>(impl_))//AND HERE I'M CONSTRUCTING <--2
{
}

but in order for this to be dependable this order MUST be from left to right. Is it guarantied by GREAT BOOK OF std:: or not? If not I can always move the second line into the body.