How to cause a length_error exception in C++ - c++

Question

I am trying to make code that throws a length_error exception. My goal is to detect and handle this exception condition. This is my attempt so far:
#include <iostream>
#include <string>
using namespace std;
int main(int argc, const char * argv[])
{
string buffer("hi");
cout<<buffer.max_size()<<endl;
try {
while(1) {
buffer.append(buffer);
}
}
catch(length_error &l) {
cout<<"Caught length_error exception: "<<l.what()<<endl;
}
catch(exception &e) {
cout<<"Caught exception: "<<e.what()<<endl;
}
return 0;
}
When I run the program I see the max size of the string is 18446744073709551599 bytes. The program continues running until all the memory is used up. Then it just goes quiet. No exception is thrown. The program is still running but its CPU usage went from 100% to around 0%.
Additional information:
OS: Mac OS 10.8.
Compiler: Clang 5.1
RAM: 16 GB

Answer 1

I believe your machine is going into virtual memory thrashing due to the memory consumption of growing your string by two characters a LOT of times.
A more effective way of getting this exception is to create a string of size max_size()+1 at the outset. Here's your code modified to do this, and (for me, at least) it throws the exception you expect instantly:
#include <iostream>
#include <string>
using namespace std;
int main(int argc, const char * argv[])
{
string buffer("hi");
cout<<buffer.max_size()<<endl;
try {
std::string blah(buffer.max_size()+1, 'X');
}
catch(length_error &l) {
cout<<"Caught length_error exception: "<<l.what()<<endl;
}
catch(exception &e) {
cout<<"Caught exception: "<<e.what()<<endl;
}
return 0;
}

Answer 2

std::length_error is only thrown in a case where a buffer size is known to exceed the container's max_size().
On a modern operating system with virtual memory, it's unlikely to request something that exceeds max_size() unless by accident (such as through negative unsigned values), and so the approach you are taking is unlikely to see this exception thrown. Instead, since you're using append, you're likely going to just use virtual memory by paging out to disk after you have exhausted your real memory -- which will slow the system down.
If you want to trigger a length_error, the easiest way would be to pass something greater than max_size() (assuming this is smaller than std::numeric_limits<std::string::size_type>::max()). You should be able to just do something like:
auto str = std::string{};
str.reserve(static_cast<std::string::size_type>(-1));
or
auto str = std::string{};
str.reserve(str.max_size()+1);