C++ VS (2019) std::string inside class created with new and memory Diagnostic Tool question

Question

EDIT:
I decide rewrite my question to more simple because nobody was understands what I wanted to "say" in previously updated question and I discovered that it is not problem with my usage of new and delete, but somewhere else.

---

If I use inside class test_mem code type_string = typeid(*this).name(); and use Diagnostic Tool while debug, and take memory snapshots under breakpoints (Memory Usage is turned on), I got this Allocations (Diff): when objects are created +4 and when destroyed -3.

When I remove inside constructor type_string = typeid(*this).name();, Allocations are same (+x -x).

Then question is, can I ignore this Allocation +4 -3 (Diagnostic Tool)`? And why those Allocations are different, it does mean string doesn't delete its chars?

Here is source code, you can try it.

#include <iostream>
#include <string>
#include <typeinfo>

class test_mem
{
 std::string type_string;
public:
 test_mem()
 {
 type_string = typeid(*this).name();
 }

 std::string get_type() { return type_string; }
};

// -----------------------------------------------

void memory_test()
{
 test_mem tm1; // use breakpoint here and take snapshot
 test_mem tm2;
 test_mem tm3;
 test_mem tm4;
} // use breakpoint here and take snapshot (Allocation is +XX)

// -----------------------------------------------

int main()
{
 memory_test();
 return 0; // use breakpoint here and take snapshot (Allocation is -XX)
}

Answer 1

Hello @Code Wanderer ,

Welcome to Microsoft Q&A forum.

If you try to test with this code cout << "" will you see an increase in the Allocations? The output content should occupy the memory.

Besides, I think you can also refer to this thread: Does a std:string always require heap memory? Also, since the codes you provided are not full code, there may be some missing of delete code.

Best Regards,
Tianyu

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Answer 2

Hello,
1. do not define the string in the class declarations, define the values in the (standard) constructor:

class test_c
{
std::string name;
public:
test_c() { name = "none"; }
//...

class obj
{
protected:
std::string type_string;
public:
obj() { type_string = ""; }

2. the classes "obj_meta" and "ob" look ok: there is one new in the constructor and one corresponding delete in the destructor.

3. in memory_test, you call new without the corresponding delete at the end of the function:

void memory_test()
{
test_c* Tc1 = new test_c("object 1", 1);
ob o1(Tc1);
// ...
delete Tc1;
}

Regards, Guido

Answer 3

It seems that the implementation of typeid allocates certain data and keeps (caches) them for future reusage. See, for example, some details inside the std_type_name.cpp file from a subfolder of "VC\Tools\MSVC", which is usually present in Visual Studio.