为什么“ unordered_map”分配内存2次？

问题描述

#include <iostream>
#include <unordered_map>

#define print(x) std::cout << x
#define println(x) std::cout << x << std::endl

void* operator new(size_t size) {
    println("Allocated: " << size << " Bytes of memory");
    return malloc(size);
}

void operator delete(void* object) {
    println("Deleted !");
    free(object);
}

template<typename T>
T input(const char* string) {
    print(string);
    T temp;
    std::cin >> temp;
    return temp;
}

int main() {
    //println(sizeof(const char*) * 4 + sizeof(int) * 4);
    std::unordered_map<const char*,int> ages;
    ages.reserve(3);
    ages["Ahmed"] = 17;
    std::initializer_list<const char*> names = {"Asem","Khalid"};
    int i = 17;
    for (const char* name : names) {
        ages[name] = i;
        i++;
    }
    for (auto age : ages) {
        println(age.first << " is " << age.second << " years old!");
    }
}

当我尝试在“发布模式”下运行此代码时，它给出以下输出：

Allocated: 16 Bytes of memory
Allocated: 64 Bytes of memory
Allocated: 16 Bytes of memory
Allocated: 16 Bytes of memory
Allocated: 16 Bytes of memory
Khalid is 18 years old!
Asem is 17 years old!
Ahmed is 17 years old!
Deleted !
Deleted !
Deleted !
Deleted !
Deleted !

我尝试调试代码，并注意到以下内容：

std::unordered_map<const char*,int> ages;

执行，将内存分配给时间，输出：

Allocated: 16 Bytes of memory
Allocated: 64 Bytes of memory

为什么会这样？，顺便说一句，当我尝试在调试模式下运行它时，它给了我2个以上的分配空间：/ 编辑：我的平台是Windows 10专业x64，我正在使用msvc

解决方法

您可以使用dbghelp库在内存分配上打印内部堆栈跟踪。这将使您深入了解stl，其中unordered_map分配内存。

#pragma comment(lib,"Dbghelp.lib")

using namespace std;


#define TRACE_MAX_STACK_FRAMES 1024
#define TRACE_MAX_FUNCTION_NAME_LENGTH 1024


int printStackTrace()
{
    void *stack[TRACE_MAX_STACK_FRAMES];
    HANDLE process = GetCurrentProcess();
    SymInitialize(process,NULL,TRUE);
    WORD numberOfFrames = CaptureStackBackTrace(0,TRACE_MAX_STACK_FRAMES,stack,NULL);
    SYMBOL_INFO *symbol = (SYMBOL_INFO *)malloc(sizeof(SYMBOL_INFO) + (TRACE_MAX_FUNCTION_NAME_LENGTH - 1) * sizeof(TCHAR));
    symbol->MaxNameLen = TRACE_MAX_FUNCTION_NAME_LENGTH;
    symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
    DWORD displacement;
    IMAGEHLP_LINE64 *line = (IMAGEHLP_LINE64 *)malloc(sizeof(IMAGEHLP_LINE64));
    line->SizeOfStruct = sizeof(IMAGEHLP_LINE64);
    for (int i = 0; i < numberOfFrames; i++)
    {
        DWORD64 address = (DWORD64)(stack[i]);
        SymFromAddr(process,address,symbol);
        if (SymGetLineFromAddr64(process,&displacement,line))
        {
            printf("\tat %s in %s: line: %lu: address: 0x%0X\n",symbol->Name,line->FileName,line->LineNumber,symbol->Address);
        }
        else
        {
            printf("\tSymGetLineFromAddr64 returned error code %lu.\n",GetLastError());
            printf("\tat %s,address 0x%0X.\n",symbol->Address);
        }
    }
    return 0;
} 

void* operator new(size_t size) {
    println("Allocated: " << size << " Bytes of memory");
    printStackTrace(); // add this
    return malloc(size);
}```