std::pmr::set_default_resource - cppreference.com (original) (raw)
If r is not null, sets the default memory resource pointer to r; otherwise, sets the default memory resource pointer to std::pmr::new_delete_resource().
The default memory resource pointer is used by certain facilities when an explicit memory resource is not supplied. The initial default memory resource pointer is the return value of std::pmr::new_delete_resource.
This function is thread-safe. Every call to std::pmr::set_default_resource synchronizes with (see std::memory_order) the subsequent std::pmr::set_default_resource and std::pmr::get_default_resource calls.
[edit] Return value
Returns the previous value of the default memory resource pointer.
[edit] Example
#include
#include
#include
#include
#include
#include
#include
class noisy_allocator : public std::pmr::memory_resource
{
void* do_allocate(std::size_t bytes, std::size_t alignment) override
{
std::cout << "+ Allocating " << bytes << " bytes @ ";
void* p = std::pmr::new_delete_resource()->allocate(bytes, alignment);
std::cout << p << '\n';
return p;
}
void do_deallocate(void* p, std::size_t bytes, std::size_t alignment) override
{
std::cout << "- Deallocating " << bytes << " bytes @ " << p << '\n';
return std::pmr::new_delete_resource()->deallocate(p, bytes, alignment);
}
bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override
{
return std::pmr::new_delete_resource()->is_equal(other);
}
};
int main()
{
constexpr int push_back_limit{16};
noisy_allocator mem;
std::pmr::set_default_resource(&mem);
{
std::cout << "Entering scope #1 (without buffer on stack)...\n";
std::cout << "Creating vector v...\n";
std::pmr::vector<std::uint16_t> v{1, 2, 3, 4};
std::cout << "v.data() @ " << v.data() << '\n';
std::cout << "Requesting more...\n";
for (int i{0}; i != push_back_limit; ++i)
{
v.push_back(i);
std::cout << "v.size(): " << v.size() << '\n';
}
std::cout << "Exiting scope #1...\n";
}
std::cout << '\n';
{
std::cout << "Entering scope #2 (with buffer on stack)...\n";
std::uint8_t buffer[16];
std::cout << "Allocating buffer on stack: " << sizeof buffer << " bytes @ "
<< static_cast<void*>(buffer) << '\n';
std::pmr::monotonic_buffer_resource mem_res{std::data(buffer), std::size(buffer)};
std::cout << "Creating vector v...\n";
std::pmr::vector<std::uint16_t> v{{1, 2, 3, 4}, &mem_res};
std::cout << "v.data() @ " << v.data() << '\n'; // equals to buffer address
std::cout << "Requesting more...\n";
for (int i{0}; i != push_back_limit; ++i)
{
v.push_back(i);
std::cout << "v.size(): " << v.size() << '\n';
}
std::cout << "Exiting scope #2...\n";
}
}
Possible output:
Entering scope #1 (without buffer on stack)... Creating vector v...
- Allocating 8 bytes @ 0x1f75c30 v.data() @ 0x1f75c30 Requesting more...
- Allocating 16 bytes @ 0x1f75c50
- Deallocating 8 bytes @ 0x1f75c30 v.size(): 5 v.size(): 6 v.size(): 7 v.size(): 8
- Allocating 32 bytes @ 0x1f75c70
- Deallocating 16 bytes @ 0x1f75c50 v.size(): 9 v.size(): 10 v.size(): 11 v.size(): 12 v.size(): 13 v.size(): 14 v.size(): 15 v.size(): 16
- Allocating 64 bytes @ 0x1f75ca0
- Deallocating 32 bytes @ 0x1f75c70 v.size(): 17 v.size(): 18 v.size(): 19 v.size(): 20 Exiting scope #1...
- Deallocating 64 bytes @ 0x1f75ca0
Entering scope #2 (with buffer on stack)... Allocating buffer on stack: 16 bytes @ 0x7fffbe9f8240 Creating vector v... v.data() @ 0x7fffbe9f8240 Requesting more...
- Allocating 64 bytes @ 0x1f75ca0 v.size(): 5 v.size(): 6 v.size(): 7 v.size(): 8 v.size(): 9 v.size(): 10 v.size(): 11 v.size(): 12 v.size(): 13 v.size(): 14 v.size(): 15 v.size(): 16
- Allocating 128 bytes @ 0x1f75cf0 v.size(): 17 v.size(): 18 v.size(): 19 v.size(): 20 Exiting scope #2...
- Deallocating 128 bytes @ 0x1f75cf0
- Deallocating 64 bytes @ 0x1f75ca0