C++17 in Detail: A Deep Dive/

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Initializing string Members from string_view

Initializing string Members from string_view

We'll continue our analysis of the example from the last chapter, this time using std::string_view instead of std::string.

We'll cover the following...

Last time, we were left with this code:

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#include <iostream>
#include <string>
#include <string_view>
using namespace std;
class UserName 
{ 
  std::string mName;
  public:
  UserName(std::string_view sv) : mName(sv) { }
  std::string_view getName(){return mName;}
};
std::string GetString() { return "some string..."; }
int main(){
  // creation from a string literal
  UserName u1{"John With Very Long Name"};
  cout << u1.getName() << endl;
  
  // creation from l-value:
  std::string s2 {"Marc With Very Long Name"};
  UserName u2 { s2 };
  cout << u2.getName() << endl;
  // use s2 later...
  
  // from r-value reference
  std::string s3 {"Marc With Very Long Name"};
  UserName u3 { std::move(s3) };
  cout << u3.getName() << endl;
  // third case is also similar to taking a return value: 
  
  UserName u4 { GetString() };
  cout << u4.getName() << endl;
}

Since the introduction of move semantics in C++11, it’s usually better, and safer to pass string as a value and then move from it.

For example:

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class UserName { 
  std::string mName;
  public:
  UserName(std::string str) : mName(std::move(str)) { }
};

Now we have the following results:

For std::string:

  • u1 - one allocation - for the input argument and then one move into the mName. It’s better than with const std::string& where we got two memory allocations in that case. And similar to the string_view approach.
  • u2 - one allocation - we have to copy the value into the argument, and then we can move from it.
  • u3 - no allocations, only two move operations - that’s better than with string_view and const string&!

When you pass std::string by value not only is the code simpler, there’s also no need to write separate overloads for rvalue references.

See the full code sample:

initializing_from_string_view.cpp

The approach with passing by value is consistent with item 41 - “Consider pass by value for copyable parameters that are cheap to move and always copied” from Effective Modern C++ by Scott Meyers. ...