Placement new? Why can't I read?

Question

5.00/5 (3 votes)

See more:

new expression - cppreference.com[^]

Useless.

Placement new operator in C++ - GeeksforGeeks[^]

Bunk.

Placement syntax - Wikipedia[^]

Written by somebody who doesn't care if it's understood by anyone.

Apparently I can't read code.
Apparently I can't copy code.

Or placement new simply does not work.

Can anyone make the code in what have you tried compile?

What I have tried:

C++

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
struct test final {
    test() {
        printf("test::test()\r\n");
    }
    ~test() {
        printf("test::~test()\r\n");
    }
};
template <typename T>
T* create(void* buf) {
    
    T* tptr = new (buf) T;  // Construct a `T` object, placing it directly into your
                            // pre-allocated storage at memory address `buf`
    return tptr;
}
int main(int argc, char** argv) {
    uint8_t ptest_buf[sizeof(test)];
    test* ptest = create<test>(ptest_buf);
    return 0;
}

Posted 8-Nov-23 21:26pm

honey the codewitch

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Comments

Maxim Kartavenkov 9-Nov-23 5:13am

Compiled correctly on MSVC

2 solutions

Solution 1

Your call to operator new is not quite correct. It goes something like:

// // new(a) type; calls operator new(sizeof(type), a); // new(a,b) type[42]; calls operator new(sizeof(type)*42, a, b); // thus // new(T) calls operator new(sizeof(T)); //

So your template should be something like:

C++

template <typename T> T* create(void* buf) { T* tptr = new (T); // Construct a new `T` struct return tptr; } // ... uint8_t ptest_buf[sizeof(test)]; // You already allocate space for the test struct test* ptest = create<test>(ptest_buf); // so what are you trying to do here?

But I am not sure what the buf parameter is for, since you return the pointer to the newly allocated structure.

Posted 8-Nov-23 22:18pm

Richard MacCutchan

Updated 8-Nov-23 23:29pm

v2

Comments

honey the codewitch 9-Nov-23 4:36am

This is the standard new operator. Not the emplace new operator. I am trying to call new on an existing buffer using the emplace new operator.

Richard MacCutchan 9-Nov-23 4:45am

Sorry, never heard of that, obviously something I need to study.

CPallini 9-Nov-23 5:22am

It is a weird thing used by us weird people.

Richard MacCutchan 9-Nov-23 5:34am

So true.

honey the codewitch 9-Nov-23 8:55am

In IoT and embedded you have different physical heaps with different sizes and performance characteristics. You have PSRAM and SRAM for example. Because of this, a user of my lib can choose the allocators to use when it needs to dynamically allocate RAM. Right now I've been very careful about RAII on anything that is the target of those allocations, but I'd rather use placement new and fire the constructors cleanly.

Richard MacCutchan 9-Nov-23 9:13am

You can add your own implementation(s) of operator new to handle the different situations, if the default does not provide for what you need. I have spent the last couple of hours working on this and it seems quite straightforward, once you understand how it is supposed to work.

honey the codewitch 9-Nov-23 9:15am

In that case it's not practical, because the user may need to select the heap to use at runtime, and will typically be using multiple heaps at the same time. Overloading new *could* get you that, if you made it call out to some switchable allocation scheme, but it's clumsier than using emplace new inside my libs

CPallini 9-Nov-23 9:49am

Well, I guessed something like that.
As for me, I did use it for C++ objects whose memory was managed by the Lua runtime.

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CPallini · Accepted Answer · 2023-11-08T23:13:00

You forgot to include the new header which provides standard library implementations of the placement new (as an alternative, you could provide your own implementations).
The following code compiles and runs fine on my Linux box.

C++

#include <cstdint>
#include <cstdlib>
#include <new>

struct test final {
    test() {
        printf("test::test()\r\n");
    }
    ~test() {
        printf("test::~test()\r\n");
    }
};




template <typename T>
T* create(void* buf) {

    T* tptr = new (buf) T;  // Construct a `T` object, placing it directly into your

                            // pre-allocated storage at memory address `buf`
    return tptr;
}



int main(int argc, char** argv)
{
    uint8_t ptest_buf[sizeof(test)];
    test* ptest = create<test>(ptest_buf);
    printf("%p\n", ptest);
}