resize or reserve

const DWORD  nNumberOfBytesToRead = 16777216;
std::string s;
s.resize( nNumberOfBytesToRead + 4096 * 2);
char* buf = &s[4096 - size_t(s.c_str()) % 4096]; //выравнивание

const DWORD  nNumberOfBytesToRead = 16777216;
std::string s;
s.reserve( nNumberOfBytesToRead + 4096 * 2);
char* buf = &s[4096 - size_t(s.c_str()) % 4096]; //выравнивание

const DWORD  nNumberOfBytesToRead = 16777216;
std::unique_ptr<char[]> sp(new char[nNumberOfBytesToRead + 4096]);
char* buf = sp.get() + nNumberOfBytesToRead % 4096; //выравнивание

const DWORD  nNumberOfBytesToRead = 16777216;   
std::unique_ptr<char[]> bufPtr = std::make_unique<char[]>(nNumberOfBytesToRead + 4096);
char* buf = bufPtr.release() + 4096 - (size_t(bufPtr.get()) % 4096);

const DWORD  nNumberOfBytesToRead = 16777216;
std::unique_ptr<char[]> bufPtr(new char[nNumberOfBytesToRead + 4096]);
char* buf = bufPtr.get() + 4096 - (size_t(bufPtr.get()) % 4096);

    std::string s;
    s.reserve( nNumberOfBytesToRead + 4096 );//resize//reserve
    std::cout << size_t(s.c_str()) << std::endl;
    char* buf = &s[4096 - size_t(s.c_str()) % 4096]; //выравнивание
    std::cout << size_t(buf) << std::endl;
    char* buf2 = buf + 1;
    std::cout << size_t(buf2) << std::endl;
    buf2[0]= 'e';
    buf2[1] = 'n';
    buf2[2] = 'd';
    buf2[3] = '\0';
    std::cout << buf2 << std::endl;

    int N = 1000000;
    auto start = std::chrono::steady_clock::now();
    for (int i = 0; i < N; i++) {
        char* p = (char*)malloc(1024);
        free(p);
    }
    auto end = std::chrono::steady_clock::now();
 
    std::cout << "malloc + free: " << (end - start).count() << std::endl;
 
    start = std::chrono::steady_clock::now();
    for (int i = 0; i < N; i++) {
        char* p = new char[1024];
        delete[] p;
    }
    end = std::chrono::steady_clock::now();
 
    std::cout << "new[] + delete[]: " << (end - start).count() << std::endl;