std::string allocate and free测试

本例写一个小的程序来进行std::string的allocate和free测试。
通过重载operator new和operator delete操作符，可以看到std::string分配和释放堆内存的全过程。
本例有一个flag new_instrumentation_on用于开启是否打印调试信息。
代码如下，
conanfile.txt

[requires]
boost/1.72.0

[generators]
cmake

CMakeLists.txt

cmake_minimum_required(VERSION 2.6)

project(optimize)
set(ENV{PKG_CONFIG_PATH} "$ENV{PKG_CONFIG_PATH}:/usr/local/lib/pkgconfig/")

set ( CMAKE_CXX_FLAGS "-pthread")
set(CMAKE_CXX_STANDARD 17)
add_definitions(-g)

include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
conan_basic_setup()

include_directories(${CMAKE_CURRENT_SOURCE_DIR}/include)
LINK_DIRECTORIES(${LINK_DIRS})

file( GLOB main_file_list ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp) 
file( GLOB source_file_list ${CMAKE_CURRENT_SOURCE_DIR}/impl/*.cpp)

foreach( main_file ${main_file_list} )
    file(RELATIVE_PATH filename ${CMAKE_CURRENT_SOURCE_DIR} ${main_file})
    string(REPLACE ".cpp" "" file ${filename})
    add_executable(${file}  ${main_file} ${source_file_list})
    target_link_libraries(${file}  ${CONAN_LIBS} pthread)
endforeach( main_file ${main_file_list})

allocator_test.cpp

#include <iostream>
#include <memory>
#include <string>

bool static new_instrumentation_on = false;

void instrument_new(bool f = true) {
    new_instrumentation_on = f;
}

// std::hex 转16进制输出
// std::dec 转10进制输出
// std::oct 转8进制输出
void* operator new(std::size_t size) {
    void *p = malloc(size);
    if(new_instrumentation_on) {
        std::cout << "new (" << size << ") allocated at 0x" << std::hex << p << std::dec << std::endl;
    }
    return p;
}

void operator delete(void *p) {
    if(new_instrumentation_on) {
        std::cout << "delete 0x" << std::hex << p << std::dec << std::endl;
    }
    free(p);
}

int string_allocator_test() {
    instrument_new(); 
    {
        puts("std::string s");
        std::string s;
        puts("try appending one char");
        s = s + 'x';
        puts("try appending 20 char C-style string");
        s = s + "12345678901234567890";
        puts("try appending s");
        s = s + s;
        puts("for");
        for(int i=0; i<30; ++i) {
            s = s + 'x';
        }  
        puts("delete s");
    }
    instrument_new(false);
    return 1;
}

int main(int argc, char* argv[]) {
    string_allocator_test();
    return EXIT_SUCCESS;
}

程序输出如下，表明如果要频繁的连接字符串，使用+= 操作符操作字符，最好先用reserve保留足够的内存，否则会一直动态分配和释放内存，影响效率。