String in cpp

Posted by chunyang on October 13, 2019
TAGS: #cpp


Strings are very common in every languages. The performance of string's methods usually have a big impact on the whole program's speed. In this blog, we will investigate common methods of a string.

Convert to and from numbers

Convert a number into a string

There are two ways:

  • to_string
  • ostringstream
int a = 3;
cout << std::to_string(a) << endl;

ostringstream os;
os << a;
cout << os.str() << endl;

Convert a number from a string

There are two ways:

  • stoll, stol, stoi, stof, stod, stold, stoul, stoull
  • istringstream
string a = "3";
int b = stoi(a);

istringstream is(a);
a >> b;

Construct a string

String is one of the sequence container like STL's vector.

  • default construct
  • repeat characters
  • from a c-style string
  • from a sequence
  • from a substring of another string
#include <string>
using namespace std;
string a;
string b(3, 'c');
string c("hello world")
string d(c.begin(), c.end());
string e(c, 3, 4); // start from the third with a count equals 4
string f("hello", 3); // start from the first with a count equals 3

Manipulate a string

Convert case

#include <string>
#include <algorithm>
string a = "hEllO"
transform(a.begin(), a.end(), a.begin(), std::tolower);
transform(a.begin(), a.end(), a.begin(), std::toupper);

Add or remove element

  • push_back, pop_back
  • append(): similar to construct a string
    • append a range, a substring, repeating characters, etc.
  • erase
    • erase(start_index, count)
    • erase(iterator)
    • erase(begin_iterator, end_iterator)
  • insert
  • substr(pos, count)
  • copy(dest, pos, count)

Search, compare, replace

  • substring search
    • find
    • rfind
  • any character search
    • find_first_of
    • find_first_not_of
    • find_last_of
    • find_last_not_of
  • compare
    • -1, 0, 1
  • replace
    • replace(pos, count1, count2, character)
    • replace(iter1, iter2, iter11, iter22)

Hash values

C++ has provide a default implementation for string and many other types.

  • bool, integers, float and double, char
  • Pointer: please be aware that, c-style string hash is calculated as a pointer, not using its content.
#include <functional>
string a = "hello";

// customization of a hash

namespace std {
    template <>
    struct hash<Type> {
        size_t operator()(const Type& v) {
            return 0; // return v's hash value

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