Written by Niels Moseley
on July 31, 2023

A string replace function for C++

Shown below is the C++ source code for a search-and-replace function. Something like this should be in the standard library but surprisingly, it isn’t. The implementation avoids using std::find for efficiency.

Note, this is not the fastest solution as each character can be checked multiple times. A much better method would be based on the KMP pattern matching.


#include <string>
#include <string_view>
#include <algorithm>

std::string replace(const std::string &text, const std::string &findText, const std::string &replaceText)
{
    if (findText.size() > text.size()) return text;
    if (findText.empty()) return text;

    std::string result;
    result.reserve(text.size());

    std::size_t idx = 0;
    std::size_t searchEndIdx = text.size() - findText.size() + 1;
    while(idx < searchEndIdx)
    {
        std::size_t searchIdx = 0;
        while(searchIdx < findText.size())
        {
            if (text.at(idx+searchIdx) != findText.at(searchIdx))
                break;  // exit, no (complete) match

            searchIdx++;
        }

        if (searchIdx == findText.size())
        {
            // found a complete match
            result.append(replaceText);
            idx += findText.size();
        }
        else
        {
            // found a partial or no match
            result += text.at(idx++);
        }
    }

    // append the last bit of the string, if needed.
    if (idx != text.size())
    {
        result.append(std::string_view(text.begin()+idx, text.end()));
    }

    return result;
}

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