There was a post on the ISOCpp blog last week about passing by const-reference or by value. The full StackOverflow post is Why do we copy then move? and another related post asks Are the days of passing by const ref over?.
First, as my friend and colleague Andy Sawyer pointed out in a conversation, taking a parameter by value leaks implementation detail out of the function so he recommends passing by const reference unless there’s a good reason not to.
Second, whilst passing by value presents the opportunity for the callee to store data using move semantics, the callee has to invoke that explicitly by calling std::move:
#include <string>
#include <iostream>
class A
{
public:
A( const std::string& s ) :
str_( s )
{
std::cout << "A: s='" << s << "', str_='" << str_ << "'\n";
}
private:
std::string str_;
};
class B
{
public:
B( std::string s ) :
str_( s )
{
std::cout << "B: s='" << s << "', str_='" << str_ << "'\n";
}
private:
std::string str_;
};
class C
{
public:
C( std::string s ) :
str_( std::move(s) )
{
std::cout << "C: s='" << s << "', str_='" << str_ << "'\n";
}
private:
std::string str_;
};
int _tmain(int argc, _TCHAR* argv[])
{
A a( "ConstReference" ); // Constructor(char*) then CopyConstructor
B b( "PassByValue" ); // Constructor(char*) then CopyConstructor
C c( "PassByValueAndMove" );// Constructor(char*) then MoveConstructor
return 0;
}
The first Jack Reacher thriller I read was One Shot. It came free with a copy of the London Evening Standard. That’s when the Standard cost 50p, before it became a free paper. On the back of that, I went on to read all the Jack Reacher stories – and One Shot wasn’t even the best. The best stories are those in which he gets members from his team of Special Investigators back together.
I must have read the Foundation novels years ago and was intrigued to spot this sequel in the book shop. It’s a brilliant read, packed full of technology, philosophy, planetary adventures and even politics. Highly recommended.
musingstudio 