How can std::string\'s rfind be used to check for a prefix and convert a substring to an integer?

Using std::string's rfind Overloads to Check for Prefix and Convert Substring to Int

To determine if a C std::string begins with a specific string and convert a substring to an integer, you can leverage the overloaded rfind function.

Checking for String Prefix

Use the rfind overload that accepts a search position parameter (pos). Setting this parameter to zero restricts the search to the beginning of the string:

<code class="cpp">std::string s = "tititoto";
if (s.rfind("titi", 0) == 0) {
  // The string s starts with the "titi" prefix.
}</code>

Converting Substring to Int

To extract a substring after a known prefix and convert it to an integer, you can use a combination of rfind and substr:

<code class="cpp">std::string arg = "--foo=98";
size_t pos = arg.rfind("--foo=");
if (pos != std::string::npos) {
  std::string fooValue = arg.substr(pos + len("--foo="));
  int value = std::stoi(fooValue);
}</code>

In this example, if arg is "--foo=98", the variable value will be assigned the integer value 98.

STL Advantage

This approach avoids the need for external libraries like Boost. It utilizes the standard string manipulations provided by the STL, which are straightforward to implement and efficient.

C 20 Simplification

In C 20 and later, the std::string and std::string_view classes introduce the starts_with method, which makes checking for a prefix even simpler:

<code class="cpp">std::string s = "tititoto";
if (s.starts_with("titi")) {
  // The string s starts with the "titi" prefix.
}</code>

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