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Effective C++ Third version, sample code compile list(14)Think carefully about copying behavior in resource-managing classes

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Effective C++: 55 Specific Ways to Improve Your Programs and DesignsThird version by Scott Mayers

https://www.informit.com/store/effective-c-plus-plus-55-specific-ways-to-improve-your-9780321334879

https://www.amazon.com/dp/0321334876

Sample code from

Yuzhen11/Effective_CPP

https://github.com/Yuzhen11/Effective_CPP


算譜(source code)


ec14.cpp

///Effective C++:  55 Specific Ways to Improve Your Programs and DesignsThird version by Scott Mayers

///https://www.informit.com/store/effective-c-plus-plus-55-specific-ways-to-improve-your-9780321334879
#include <iostream>
#include <cstdlib>
#include <mutex>
#define Mutex mutex

using namespace std;

const char * msg ="Item 14: Think carefully about copying behavior in resource-managing classes";

/// From here "Effective C++

//In previous item, we see that heap-based resources can be managed using shared_ptr. Not all resources are heap-based.

//C API:

void lock(Mutex* pm); // lock mutex pointed to by pm
void unlock(Mutex* pm); // unlock the mutex
//RAII: Resources are acquired during construction and released during destruction.

class Lock {
public:
explicit Lock(Mutex* pm): mutexPtr(pm) {
lock(mutexPtr); // acquire resource
}
~Lock() {
unlock(mutexPtr); // release resource
}
private:
Mutex* mutexPtr;
};

void f() {
Mutex m;
{
Lock ml(&m);
}

//What about the copying behavior?

Lock ml1(&m);
Lock ml2(&ml1); // copy ml1 to ml2, what should happen here?
}
//Prohibit copying

namespace A {
class Lock : private Uncopyable { // see Item 6
public:
// ...
};
//reference-count the underlying resource Copying the RAII object should increment the count of the number of objects.
//deleter should be defined
}

namespace B {
class Lock {
public:
explicit Lock(Mutex* pm): mutexPtr(pm, unlock) // deleter added
{
lock(mutexPtr.get());
}
private:
std::shared_ptr<Mutex> mutexPtr;
};
//Copying the underlying resource
//Make sure a deep copy. For example, STL.
}

//Transfer ownership of the underlying resource
//Example: auto_ptr

//Things to Remember

//Copying an RAII object entails copying the resource it manages, so the copying behavior of the resource determines the copying behavior of the RAII object.
//Common RAII class copying behaviors are disallowing copying and performing reference counting, but other behaviors are possible.

/// to here from "Effective C++"

int main() {
f();
cout << msg << endl;
return EXIT_SUCCESS;
}



編纂・実行結果(compile and go)


cpa.sh

$ ../c1.sh ec14

$ clang++ ec14.cpp -std=c++2a -Wall
ec14.cpp:39:6: error: no matching constructor for initialization of 'Lock'
Lock ml2(&ml1); // copy ml1 to ml2, what should happen here?
^ ~~~~
ec14.cpp:22:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from
'Lock *' to 'const Lock' for 1st argument; remove &
class Lock {
^
ec14.cpp:24:14: note: candidate constructor not viable: no known conversion from 'Lock *' to 'std::__1::mutex *'
for 1st argument
explicit Lock(Mutex* pm): mutexPtr(pm) { lock(mutexPtr); } // acquire resource
^
ec14.cpp:44:22: error: expected class name
class Lock : private Uncopyable { // see Item 6
^
2 errors generated.


課題(agenda)

1)役に立つまたは意味のあるコンパイルエラーを取る方法

2)役に立つまたは意味のある関数、クラスの呼び出し方

3)役に立つまたは意味のある出力


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文書履歴(document history)

ver. 0.10 初稿 20180622