C++ STL algorithms: get pointers of elements in container

Question

I have a function that is taking two pointers as parameters.

bool function(T* a, T* b);

And a container (vector).

vector<T> vector;

I wish to use an algorithm of the STL (the for_each) to apply the function on the elements of the vector. However, I don't know if the pointers of the elements will go automatically in the function as parameters. For example, will

for_each(vector.begin(), vector.end(), function(/*something here?*/))

work, or I need to go with a loop (the new for each in loop of the C++11) ?

I would honestly think the for_each loop is an akward way to do this, but what is it exactly you are trying to do? Are you wanting for each element of the vector[i], to be put through function(vector[i],vector[i+1]) — Lilith Daemon
– Lilith Daemon, Commented Apr 8, 2016 at 0:26
Wanted to know if there is a way with algorithms, but yes, I don't have a choice and need the loop. — wayland700
– wayland700, Commented Apr 8, 2016 at 0:55

Sam Varshavchik · Accepted Answer · 2016-04-08 00:29:33Z

3

You cannot use std::for_each() with your function for at least two reasons:

std::for_each passes a single parameter to the lamba, your function() takes two parameters.
std::for_each passes a reference to each member of the container, and not a pointer.

answered Apr 8, 2016 at 0:29

Sam Varshavchik

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4 Comments

Peter Over a year ago

It is possible to write a wrapper function which accepts a reference, obtains pointer to a couple of objects based on that reference, and passes the pointers to function().

Sam Varshavchik Over a year ago

Turning a reference into a pointer is straightforward (I'll let you in on a little secret: it usually takes exactly 0 CPU cycles to do that). However, a reference is a reference to a single object, not a "couple of objects".

Peter Over a year ago

I used the words "based on" for a reason. There is nothing stopping the wrapper making an assumption that the reference it receives is an element of a std::vector (as is the case in this question). Given the address of one element, it is then possible to reason about the address of other elements.

Sam Varshavchik Over a year ago

Sure, but only if you're careful. If you ass-ume that the other object is the one after what for_each() gives you, and you blindly use for_each() on the entire vector, you might spend a lot of time trying to figure out why your code crashes, every time.

Lilith Daemon · Accepted Answer · 2016-04-08 00:29:00Z

1

Try utilizing a normal for loop for solving this problem.

vector<T> v;
for(size_t i(0); i < v.size()-1; ++i)
{
    function(&v[i], &v[i+1]);
}

This will pass as the values an address to elements i and i+1 of v which of type T.

answered Apr 8, 2016 at 0:29

Lilith Daemon

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