Replace part of a string with another string

Is it possible in C++ to replace part of a string with another string?

Basically, I would like to do this:

QString string("hello $name");
string.replace("$name", "Somename");

But I would like to use the Standard C++ libraries.

Solution 1:

There's a function to find a substring within a string (find), and a function to replace a particular range in a string with another string (replace), so you can combine those to get the effect you want:

bool replace(std::string& str, const std::string& from, const std::string& to) {
    size_t start_pos = str.find(from);
    if(start_pos == std::string::npos)
        return false;
    str.replace(start_pos, from.length(), to);
    return true;
}

std::string string("hello $name");
replace(string, "$name", "Somename");

In response to a comment, I think replaceAll would probably look something like this:

void replaceAll(std::string& str, const std::string& from, const std::string& to) {
    if(from.empty())
        return;
    size_t start_pos = 0;
    while((start_pos = str.find(from, start_pos)) != std::string::npos) {
        str.replace(start_pos, from.length(), to);
        start_pos += to.length(); // In case 'to' contains 'from', like replacing 'x' with 'yx'
    }
}

Solution 2:

With C++11 you can use std::regex like so:

#include <regex>
...
std::string string("hello $name");
string = std::regex_replace(string, std::regex("\\$name"), "Somename");

The double backslash is required for escaping an escape character.

Solution 3:

std::string has a replace method, is that what you are looking for?

You could try:

s.replace(s.find("$name"), sizeof("$name") - 1, "Somename");

I haven't tried myself, just read the documentation on find() and replace().

Solution 4:

To have the new string returned use this:

std::string ReplaceString(std::string subject, const std::string& search,
                          const std::string& replace) {
    size_t pos = 0;
    while ((pos = subject.find(search, pos)) != std::string::npos) {
         subject.replace(pos, search.length(), replace);
         pos += replace.length();
    }
    return subject;
}

If you need performance, here is an optimized function that modifies the input string, it does not create a copy of the string:

void ReplaceStringInPlace(std::string& subject, const std::string& search,
                          const std::string& replace) {
    size_t pos = 0;
    while ((pos = subject.find(search, pos)) != std::string::npos) {
         subject.replace(pos, search.length(), replace);
         pos += replace.length();
    }
}

Tests:

std::string input = "abc abc def";
std::cout << "Input string: " << input << std::endl;

std::cout << "ReplaceString() return value: " 
          << ReplaceString(input, "bc", "!!") << std::endl;
std::cout << "ReplaceString() input string not modified: " 
          << input << std::endl;

ReplaceStringInPlace(input, "bc", "??");
std::cout << "ReplaceStringInPlace() input string modified: " 
          << input << std::endl;

Output:

Input string: abc abc def
ReplaceString() return value: a!! a!! def
ReplaceString() input string not modified: abc abc def
ReplaceStringInPlace() input string modified: a?? a?? def