C++ std::string

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Strings are objects that represent sequences of characters. The standard string class provides a simple, safe and versatile alternative to using explicit arrays of chars when dealing with text and other sequences of characters. The C++ string class is part of the std namespace and was standardized in 1998.


  • // Empty string declaration

    std::string s;

  • // Constructing from const char* (c-string)

    std::string s("Hello");

    std::string s = "Hello";

  • // Constructing using copy constructor

    std::string s1("Hello");

    std::string s2(s1);

  • // Constructing from substring

    std::string s1("Hello");

    std::string s2(s1, 0, 4); // Copy 4 characters from position 0 of s1 into s2

  • // Constructing from a buffer of characters

    std::string s1("Hello World");
    std::string s2(s1, 5); // Copy first 5 characters of s1 into s2

  • // Construct using fill constructor (char only)

    std::string s(5, 'a'); // s contains aaaaa

  • // Construct using range constructor and iterator

    std::string s1("Hello World");

    std::string s2(s1.begin(), s1.begin()+5); // Copy first 5 characters of s1 into s2


Before using std::string, you should include the header string, as it includes functions/operators/overloads that other headers (for example iostream) do not include.

Using const char* constructor with a nullptr leads to undefined behavior.

std::string oops(nullptr);
std::cout << oops << "\n";

The method at throws an std::out_of_range exception if index >= size().

The behavior of operator[] is a bit more complicated, in all cases it has undefined behavior if index > size(), but when index == size():

  1. On a non-const string, the behavior is undefined;
  2. On a const string, a reference to a character with value CharT() (the null character) is returned.
  1. A reference to a character with value CharT() (the null character) is returned.
  2. Modifying this reference is undefined behavior.

Since C++14, instead of using "foo", it is recommended to use "foo"s, as s is a user-defined literal suffix, which converts the const char* "foo" to std::string "foo".

Note: you have to use the namespace std::string_literals or std::literals to get the literal s.

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