C++ 代码中的内存泄漏

Question

i'm writting a code in Visual Studio using c++. My test say that i have memory leaks. I dont understand why.

here is the error:

Detected memory leaks! Dumping objects -> e:\\gbm\\inf1005c\\td6\\exercice1\\exercice1.cpp(175) : {1417} normal block at 0x0073C7C8, 0 bytes long. Data: <>

thanks a lot.

please note that colletion is a struct and that the line with the memory leak is this one: nouvelleCollection.livres = new Livre*[nTitresPareils];

Collection retrouverLivresParNom(const wstring& titre, const Collection& collection) // Mettre les bons paramètres.

{ int nTitresPareils = 0;

// Retrouver les livres dans la collection dont le titre correspond à la recherche
for (int i = 0; i < collection.nLivres; i++){
    bool trouve = (wcsstr(collection.livres[i]->titre, titre.c_str()) != nullptr);
    if (trouve)
        nTitresPareils ++;
}

// Allouer l'espace qui contiendra le tableau des livres trouvés
Collection nouvelleCollection;
nouvelleCollection.livres = new Livre*[nTitresPareils]; 

// Copier les pointeurs vers les livres trouvés
int compteur = 0;

for (int i = 0; i < collection.nLivres; i++){
    bool trouve = (wcsstr(collection.livres[i]->titre, titre.c_str()) != nullptr);
    if (trouve){
        nouvelleCollection.livres[compteur] = collection.livres[i];
        compteur++;
    }
}

nouvelleCollection.nLivres = nTitresPareils;
nouvelleCollection.nLivresAlloues = nTitresPareils;

// Retourner le nombre de livres trouvés
return nouvelleCollection;

}

Answer 1

First you need to make sure that the destructor of Collection ( Collection::~Collection() ) calls delete[] on the livres member. This will prevent your memory leak. Even structs can have destructors.

However, when you do that, you also have to make another fix. The default copy constructor will do a shallow copy of the members of that class, so the pointer to the array of books will be copied but not the whole array.

Because you're returning the Collection by value, then the local copy of the Collection will be destroyed upon return. When that happens the livres array will be deallocated and the Collection variable to which the return value is assigned in the calling function will have a null livres pointer or worse a dangling pointer.

So you also need to do one of two things:

1) You need to create your Collection on the heap using new Collection and return it by pointer ( Collection* )

OR

2) You need to create a copy constructor in Collection to do a deep copy of the list of books ( livres ).

The comments on your question are good too, consider using a std::vector instead of allocating your own array. If you use a std::vector, you don't have to worry about specially handling its destruction or copying.

Bonne Chance!