Question

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C++'s “placement new”
Help with a c++ statement

I work in a product, where most of the modules have been written in C. Among them, one or two modules are written in C++. I find the below code in a C++ module, which I could not understand what is going on.

a = (char *) malloc (size);
b = new (a) MyClass();

Could someone explain me how a pointer allocated by malloc is used for new operator? Is it legitimate?

Thanks!

Answer 1

b = new (a) MyClass(); is called placement new and it constructs new object of type MyClass on a pre-allocated memory (memory that pointer a points to).

You should also check this question: What uses are there for "placement new"?