C++ Tips : std::vector

C++ Tips

std::vector

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std::vector

template<T, A = std::allocator<T>>
class Vector 
{
public:
	void push_back(const T&); 	// l-value
	void push_back(T&&); 		// r-value

	template<typename ...Args>
	void emplace_back(Args&& ...args); // Args&& universal ref
}

• Default allocator used by all STL containers if no user-defined provided.
• Dynamic and contiguous.
• Fast random access. operator[] , .at()
• Add elements via push_back() or emplace_back()
• If vector is destroyed, then elements dtor will also be called.
  • Valid for all containers.

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push_back() vs emplace_back()

• push_back()
  • Takes an already constructed object.
  • Rollback if push_back() throws. (No insertion.)
  • No return value
• emplace_back()
  • Forwards arguments to the constructor of the class using std::forward
  • Object is constructed through allocator_traits::construct.

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Tip-01

• Non-member iterator func == Member iterator func
  • Meaning, std::crbegin(vec) is equal to vec.crbegin()
• size() == 0 for all moved containers

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Tip-02

class A { A(int); }

vector<A> avec(100);
//syntax-error : A does not have def ctor

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Tip-03

vector<int> x(10);
// x.size() -> 10

vector<int> y{10};
// y.size() -> 1, {} is a initializer-list

vector<int> z(10,5);
// size() -> 10 and all elements are 5

vector<int> t{10,5};
// size() -> 2 and elements are 10 and 5

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Tip-04 : CTAD

• vector supports CTAD (Class Template Argument Deduction)

vector ivec{1,3,5};
// equals to -> vector<int> ivec{1,3,5};

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Tip-05 : Different containers can’t convert implicitly

std::list<int> x {1,3,5};

std::vector<int> y { x };
// syntax-error

std::vector<int> y { x.begin(), x.end() };
// OK

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Tip-06

int a[] {1,3,5};

// before modern c++
std::vector<int> ivec( a, a + sizeof(a) / sizeof(*a) );

// after modern c++
std::vector<int> ivec2( std::begin(a), std::end(a) );

// std::size(y) == y.size()
// no distinct

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Tip-07 : Prefer stl algorithms instead of raw loop

for(auto iter = vec.begin(); iter != vec.end(); ++iter) {...}

for(auto x : vec) {...}
// x -> *iter

for(size_t i{}; i < vec.size(); ++i) 
{
	vec.at(i);
	// if index is invalid, exception is thrown
}

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Tip-07

std::vector<std::string> x { "A", "B" };
std::vector<std::string> y;

y.push_back( std::move(x[0]) );
y.push_back( std::move(x[1]) );

std::cout << x.size() << '\n';
// x.size() -> 2
std::cout << (x[0].empty() ? "EMPTY" : x[0]);
// Output : EMPTY

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vector::resize

vector<int> x{1,2,3};

x.resize(5); // 1,2,3,0,0

x.resize(2); // 1,2

x.resize(6,4); // 1,2,4,4,4,4

x.resize(0) // clears the vector
// x.clear() == x.resize(0)

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vector::assign

vector<char> c;
c.assign(5, 'a');

string s(6, 'b');

c.assign(s.begin(), s.end());
// b, b, b, b, b, b

c.assign( {'a', 'b' });
// a, b

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vector::insert

• returns iterator that points inserted value

vector<int> i(2,10); // 10, 10

i.insert(i.begin(), 25); // 25, 10, 10

i.insert(i.begin(), 2, 50); // 50, 50, 25, 10, 10

i.insert(i.end(), {1,2,3}); // 50, 50, 25, 10, 10, 1, 2, 3

int a[] {4,5,6};

i.insert(i.begin(), a, a + size(a));
// 50, 50, 25, 10, 10, 1, 2, 3, 4, 5, 6

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vector::erase

• returns iterator pointing that followed the last element erased.

vector<int> x{1,2,3,4,5};

// erase the first 3 elements:
x.erase ( x.begin(), x.begin() + 3 );

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vector::swap

vector<string> a (100, "A");
vector<string> b (100, "B");

std::swap(a,b); // same with a.swap(b);
// no string copy. 
// just pointers swapped.

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vector::pop_back()

• O(1)
• Removes the last element

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vector::max_size

• O(1)
• Returns the maximum num of elems the container is able to hold.