C++ High Performance/

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Pass by Value When Applicable

Pass by Value When Applicable

Learn about the usage of pass-by value in different cases.

We'll cover the following...

Passing by value: when and why to use it

Consider a function that converts a std::string to lowercase. In order to use the move-constructor where applicable, and the copy-constructor otherwise, it may seem like two functions are required:

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// Argument s is a const reference
auto str_to_lower(const std::string& s) -> std::string { 
    auto clone = s;
    for (auto& c: clone) c = std::tolower(c);
        return clone;
}
// Argument s is an rvalue reference
auto str_to_lower(std::string&& s) -> std::string { 
    for (auto& c: s) c = std::tolower(c);
        return s;
}
TEST(MoveSemantics, StrToLower) {
  std::cout << "tolower\n";
  auto upper = std::string{"ABC"};
  auto lower = str_to_lower(std::move(upper));
  ASSERT_TRUE(upper.empty());
  ASSERT_FALSE(lower.empty());
}

However, by taking the std::string by value instead, we can write one function that covers both cases:

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auto str_to_lower(std::string s) -> std::string {
  for (auto& c: s) c = std::tolower(c);
  return s;
}

Let's see why this implementation of str_to_lower() avoids unnecessary copying where possible. When passed a regular variable, shown as follows, the content of str is copy-constructed into ...