problem with string sequences

[alexs]

There seems to be a problem in implementing out parameter for string sequences:
void
IceInternal::BasicStream::read(vector<string>& v)
{
Int sz;
readSize(sz);
while(sz--)
{
string s;
read(s);
v.push_back(s);
}
}

Vector is not cleared, therefore it just keeps growing, causing memory problems in client code like :

Test::AStruct input,output;
for (int i=0; i<num_elems; ++i) {
input.iseq.push_back(i);
input.sseq.push_back("abcd");
}


for (int i=0;i<num_calls;++i) {
twoway->foo(input,output);
}

Time after = Time::now();

If we change BasicStream::read as :

void
IceInternal::BasicStream::read(vector<string>& v)
{
Int sz;
readSize(sz);
v.clear();
v.reserve(sz);
while(sz--)
{
string s;
read(s);
v.push_back(s);
}
}

problem goes away.

[mes]

Welcome!

Thanks for the bug report, this will be corrected in the next release.

Take care,
- Mark

[TiB]

Hello,

I had been working a bit on improving performance when passing lots of string sequences and I came up with another version which might be of some interest:

void
IceInternal::BasicStream::read(vector<string>& v)
{
    Int sz;
    readSize(sz);

    std::vector<std::string> result(sz);
    std::vector<std::string>::iterator it = result.begin();
    std::vector<std::string>::iterator const end = result.end();
    
    for(; it != end; ++it)
    {
	read(*it);
    }
    v.swap(result);
}

This gave me a significant performance improvement (on my platform RH9 + gcc3.2.2) and has the advantage of leaving the vector in a consistent state if an exception is thrown.

Cheers,
Bertrand.

[marc]

Interesting idea, thanks for the suggestion! We will try this.

[rdilipk]

The swap trick is mentioned in Effective STL by Scott Meyers to reduce the size of the vector.