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Since std::basic_string is a container of characters, this rule is a specific instance of CTR51-CPP. Use valid references, pointers, and iterators to reference elements of a container. As a container, it supports iterators just like other containers in the Standard Template Library. However, the std::basic_string template class has unusual invalidation semantics. According to the The C++ Standard, [string.require], paragraph 5 [ISO/IEC 14882-2014], states the following:

References, pointers, and iterators referring to the elements of a basic_string sequence may be invalidated by the following uses of that basic_string object:

• As an argument to any standard library function taking a reference to non-const basic_string as an argument.
• Calling non-const member functions, except operator[], at, front, back, begin, rbegin, end, and rend.

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Do not use an invalidated reference, pointer, or iterator because doing so results in undefined behavior.

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In this noncompliant code example, data is invalidated after the call to replace(), and so its use in g() is undefined behavior:.

#include <iostream>
#include <string>
 
extern void g(const char *);
 
void f(std::string &exampleString) {
  const char *data = exampleString.data();
  // ...
  exampleString.replace(0, 2, "bb");
  // ...
  g(data);
}

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In this compliant solution, the pointer to exampleString's internal buffer is not generated until after the modification from replace() has completed:.

#include <iostream>
#include <string>

extern void g(const char *);

void f(std::string &exampleString) {
  // ...
  exampleString.replace(0, 2, "bb");
  // ...
  g(exampleString.data());
}

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