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If an exception is thrown in the constructor, the best way is to prevent memory leakage?
2022-07-08 00:10:00 【Big mulberry security team】
If an exception is thrown in the constructor , Destructors will not be called ( Simple class , No inheritance ). therefore , If an exception is thrown in the constructor , And there are some chances that the heap memory has not been cleared . So what's the best thing to do here ?
If you avoid “ bare ” resources ( Such as bare pointer , Bare mutexes, etc ), And include all the contents in the appropriate RAII Behavior in a container or class , So even if there are exceptions , There will be no problem as you described .
in other words , Do not get raw resources in the constructor . Instead, create one that follows itself RAII Instance of the object . such , Even if your constructor fails ( That is, create the constructor of the instance ), The destructor of the initialization object will be called .
therefore , It's not a good idea :
#include<iostream>
#include<stdexcept>
struct Bad {
Bad() {
double *x = new double;
throw(std::runtime_error("the exception was thrown"));
}
~Bad() {
delete x;
std::cout<<"My destructor was called"<<std::endl;
}
double *x;
};
int main() {
try {
Bad bad;
} catch (const std::exception &e) {
std::cout<<"We have a leak! Let's keep going!"<<std::endl;
}
std::cout<<"Here I am... with a leak..."<<std::endl;
return 0;
}
Output :
We have a leak! Let's keep going!
Here I am... with a leak...
An example of correction :
#include<iostream>
#include<stdexcept>
struct Resource {
Resource() {
std::cout<<"Resource acquired"<<std::endl;
}
~Resource() {
std::cout<<"Resource cleaned up"<<std::endl;
}
};
struct Good {
Good() {
std::cout<<"Acquiring resource"<<std::endl;
Resource r;
throw(std::runtime_error("the exception was thrown"));
}
~Good() {
std::cout<<"My destructor was called"<<std::endl;
}
};
int main() {
try {
Good good;
} catch (const std::exception &e) {
std::cout<<"We DO NOT have a leak! Let's keep going!"<<std::endl;
}
std::cout<<"Here I am... without a leak..."<<std::endl;
return 0;
}
Output :
Acquiring resource
Resource acquired
Resource cleaned up
We DO NOT have a leak! Let's keep going!
Here I am... without a leak...
My opinion is as follows : Try to encapsulate all resources that need to be liberated into classes that the constructor does not throw , Destructors correctly release resources . then , In other classes that the destructor may throw , Just create an instance of the wrapper resource , And it will ensure that the destructor of the obtained resource wrapper will be cleaned up .
The following may be a better example :
#include<mutex>
#include<iostream>
#include<stdexcept>
// a program-wIDe mutex
std::mutex TheMutex;
struct Bad {
Bad() {
std::cout<<"Attempting to get the mutex"<<std::endl;
TheMutex.lock();
std::cout<<"Got it! I'll give it to you in a second..."<<std::endl;
throw(std::runtime_error("Ooops,I threw!"));
// will never get here...
TheMutex.unlock();
std::cout<<"There you go! I released the mutex!"<<std::endl;
}
};
struct ScopedLock {
ScopedLock(std::mutex& mutex)
:m_mutex(&mutex) {
std::cout<<"Attempting to get the mutex"<<std::endl;
m_mutex->lock();
std::cout<<"Got it! I'll give it to you in a second..."<<std::endl;
}
~ScopedLock() {
m_mutex->unlock();
std::cout<<"There you go! I released the mutex!"<<std::endl;
}
std::mutex* m_mutex;
};
struct Good {
Good() {
ScopedLock autorelease(TheMutex);
throw(std::runtime_error("Ooops,I threw!"));
// will never get here
}
};
int main() {
std::cout<<"Create a Good instance"<<std::endl;
try {
Good g;
} catch (const std::exception& e) {
std::cout<<e.what()<<std::endl;
}
std::cout<<"Now,let's create a Bad instance"<<std::endl;
try {
Bad b;
} catch (const std::exception& e) {
std::cout<<e.what()<<std::endl;
}
std::cout<<"Now,let's create a whatever instance"<<std::endl;
try {
Good g;
} catch (const std::exception& e) {
std::cout<<e.what()<<std::endl;
}
std::cout<<"I am here despite the deadlock..."<<std::endl;
return 0;
}
Output ( use gcc 4.8.1 compile , Use -std = c 11):
Create a Good instance
Attempting to get the mutex
Got it! I'll give it to you in a second...
There you go! I released the mutex!
Ooops,I threw!
Now,let's create a Bad instance
Attempting to get the mutex
Got it! I'll give it to you in a second...
Ooops,let's create a whatever instance
Attempting to get the mutex
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