Optimisation, function returns, STL containers

[Simon Elliott]

What optimisation do compilers typically provide when passing STL
containers around?

For example, if I do something like this:

struct Tbloggs
{
std::string s1;
};
typedef std::vector<Tbloggs> TbloggsList;
typedef std::vector<TbloggsList> TbloggsListContainer;

TbloggsList getBloggsList(void)
{
TbloggsList tempBloggsList;
Tbloggs b1;
tempBloggsList.push_back(b1);
return(tempBloggsList);
}

TbloggsListContainer myBloggsListContainer;
TbloggsList myBloggsList = getBloggsList();
myBloggsListContainer.push_back(myBloggsList);

In theory there's a whole lot of unnecessary copying going on here, in
the "push_back"s and the getBloggsList() return. How much of this is the
compiler allowed to optimise away?

 

[Victor Bazarov]

What optimisation do compilers typically provide when passing STL
containers around?

The same as for any other classes. That's the whole point.

For example, if I do something like this:

struct Tbloggs
{
std::string s1;

Supposedly 'std::string' has been defined here...

};
typedef std::vector<Tbloggs> TbloggsList;
typedef std::vector<TbloggsList> TbloggsListContainer;

TbloggsList getBloggsList(void)
{
TbloggsList tempBloggsList;
Tbloggs b1;
tempBloggsList.push_back(b1);
return(tempBloggsList);
}

TbloggsListContainer myBloggsListContainer;
TbloggsList myBloggsList = getBloggsList();
myBloggsListContainer.push_back(myBloggsList);

In theory there's a whole lot of unnecessary copying going on here, in
the "push_back"s and the getBloggsList() return. How much of this is the
compiler allowed to optimise away?

The construction of 'myBloggList' object is permitted to happen
"in place" (see 12.8/15). It's called "return value optimisation".

Victor

 

[lilburne]

The construction of 'myBloggList' object is permitted to happen
"in place" (see 12.8/15). It's called "return value optimisation".

Though it won't happen in assignments, so you'll need to be
carefull of any other statements like:

myBloggsList = getBloggsList();

which isn't a construction. But you should be aware of this
with any object that is returned by value.