The string class

Introduction

In C, text means managing arrays of characters by hand, with all the bugs that brings. C++ gives you std::string, which grows as needed and works with the operators you would expect. It is one of the biggest everyday improvements C++ makes.

How it works

Characters. A range-based loop goes through the characters. for (char& c : s) lets you change them. toupper(c) and tolower(c) come from <cctype>.

Not found. find returns the special value string::npos if there is no match, so always compare with that before using the result.

String streams. <sstream> lets you treat a string as if it were input. It is the simplest way of splitting a line into words:

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main() {
    string line = "the quick brown fox";
    istringstream in(line);
    string word;
    int count = 0;
    while (in >> word) {
        count++;
    }
    cout << count << " words" << endl;
    ostringstream out;
    out << count * 2 << "!";
    cout << out.str() << endl;
    return 0;
}

ostringstream works the other way round, building text with <<.

Syntax

string s = "hello";
s += " world";
if (s.find("wor") != string::npos) { ... }
int n = stoi("42");
string t = to_string(n);

Example

#include <iostream>
#include <string>
using namespace std;

int main() {
    string city = "Dhaka";
    cout << city.size() << " " << city[0] << endl;
    city += " City";
    cout << city.substr(0, 5) << endl;
    cout << city.find("City") << endl;
    string other = "Dhaka City";
    cout << (city == other) << endl;
    return 0;
}

Line by line

1. city.size() is 5, and city[0] is the first character.
2. += adds to the end, and the string grows by itself.
3. substr(0, 5) takes five characters from the start.
4. find gives the position where "City" begins.
5. == compares the text, and prints 1 for true.

Expected result

5 D
Dhaka
6
1

In the real world

Almost every program handles text, whether it is configuration, messages or file names. Using std::string rather than character arrays removes a whole class of security bugs.

Common mistakes

• Not checking for string::npos before using the result of find.
• Using s[i] past the end. It is not checked, so use s.at(i) if you are unsure.
• Using stoi on text that is not a number. It throws an exception.
• Adding two quoted texts. "a" + "b" is an error, because neither is a string yet.

Best practices

• Pass strings to functions by const string&.
• Use istringstream to split lines.
• Prefer string to character arrays in all new code.

Summary

std::string handles its own memory and supports +, ==, substr, find and more. find signals failure with string::npos, and string streams split and build text.

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