Top stl frequently asked interview questions

Why can't I make a vector of references?

When I do this:

std::vector<int> hello;

Everything works great. However, when I make it a vector of references instead:

std::vector<int &> hello;

I get horrible errors like "error C2528: 'pointer' : pointer to reference is illegal".

I want to put a bunch of references to structs into a vector, so that I don't have to meddle with pointers. Why is vector throwing a tantrum about this? Is my only option to use a vector of pointers instead?

Source: (StackOverflow)

How to find if a given key exists in a C++ std::map

I'm trying to check if value is in a map and somewhat can't do it:

typedef map<string,string>::iterator mi;
map<string, string> m;
m.insert(make_pair("f","++--"));
pair<mi,mi> p = m.equal_range("f");//I'm not sure if equal_range does what I want
cout << p.first;//I'm getting error here

so how can I print what is in p?

Source: (StackOverflow)

How do I print the elements of a C++ vector in GDB?

I want to examine the contents of a std::vector in GDB, how do I do it? Let's say it's a std::vector<int> for the sake of simplicity.

Source: (StackOverflow)

Why are Standard iterator ranges [begin, end) instead of [begin, end]?

Why does the Standard define end() as one past the end, instead of at the actual end?

Source: (StackOverflow)

Why is it wrong to use std::auto_ptr<> with standard containers?

Why is it wrong to use std::auto_ptr<> with standard containers?

Source: (StackOverflow)

How to erase element from std::vector<> by index?

I have a std::vector, and I want to delete the n'th element. How do I do that?

std::vector<int> vec;

vec.push_back(6);
vec.push_back(-17);
vec.push_back(12);

vec.erase(???);

Please help!

Source: (StackOverflow)

Throwing the fattest people off of an overloaded airplane.

Let's say you've got an airplane, and it is low on fuel. Unless the plane drops 3000 pounds of passenger weight, it will not be able to reach the next airport. To save the maximum number of lives, we would like to throw the heaviest people off of the plane first.

And oh yeah, there are millions of people on the airplane, and we would like an optimal algorithm to find the heaviest passengers, without necessarily sorting the entire list.

This is a proxy problem for something I'm trying to code in C++. I would like to do a "partial_sort" on the passenger manifest by weight, but I don't know how many elements I'm going to need. I could implement my own "partial_sort" algorithm ("partial_sort_accumulate_until"), but I'm wondering if there's any easier way to do this using standard STL.

Source: (StackOverflow)

What's the most efficient way to erase duplicates and sort a vector?

I need to take a C++ vector with potentially a lot of elements, erase duplicates, and sort it.

I currently have the below code, but it doesn't work.

vec.erase(
      std::unique(vec.begin(), vec.end()),
      vec.end());
std::sort(vec.begin(), vec.end());

How can I correctly do this?

Additionally, is it faster to erase the duplicates first (similar to coded above) or perform the sort first? If I do perform the sort first, is it guaranteed to remain sorted after std::unique is executed?

Or is there another (perhaps more efficient) way to do all this?

Source: (StackOverflow)

C++ STL Vectors: Get iterator from index?

So, I wrote a bunch of code that accesses elements in an stl vector by index[], but now I need to copy just a chunk of the vector. It looks like vector.insert(pos, first, last) is the function I want... except I only have first and last as ints. Is there any nice way I can get an iterator to these values?

Source: (StackOverflow)

Remove spaces from std::string in C++

What is the preferred way to remove spaces from a string in C++? I could loop through all the characters and build a new string, but is there a better way?

Source: (StackOverflow)

How to check that an element is in a std::set?

How do you check that an element is in a set?

Is there a simpler equivalent of the following code:

myset.find(x) != myset.end()

Source: (StackOverflow)

What's this STL vs. "C++ Standard Library" fight all about? [closed]

Someone brought this article to my attention that claims (I'm paraphrasing) the STL term is misused to refer to the entire C++ Standard Library instead of the parts that were taken from SGI STL.

(...) it refers to the "STL", despite the fact that very few people still use the STL (which was designed at SGI).

Parts of the C++ Standard Library were based on parts of the STL, and it is these parts that many people (including several authors and the notoriously error-ridden cplusplus.com) still refer to as "the STL". However, this is inaccurate; indeed, the C++ standard never mentions "STL", and there are content differences between the two.

(...) "STL" is rarely used to refer to the bits of the stdlib that happen to be based on the SGI STL. People think it's the entire standard library. It gets put on CVs. And it is misleading.

I hardly know anything about C++'s history so I can't judge the article's correctness. Should I refrain from using the term STL? Or is this an isolated opinion?

Source: (StackOverflow)

Iteration over std::vector: unsigned vs signed index variable

What is the correct way of iterating over a vector in C++?

Consider these two code fragments, this one works fine:

for (unsigned i=0; i < polygon.size(); i++) {
    sum += polygon[i];
}

and this one:

for (int i=0; i < polygon.size(); i++) {
    sum += polygon[i];
}

which generates warning: comparison between signed and unsigned integer expressions.

I'm new in the world of C++, so the unsigned variable looks a bit frightening to me and I know unsigned variables can be dangerous if not used correctly, so - is this correct?

Source: (StackOverflow)