const std：：string：：size（）比优化级别 2 中应有的要大得多-解网

问：

定义：

const std::string a_class::dft_col_name{"NONE"};

用法：

在 a_class：：function（）中

只要循环一次，我是 0

for (int i = 0; i < result->colnum; ++i) {
    result->pcolname[(i + 1) * SQLCOLLEN - 1] = '\0';
    auto colname_len = strlen(result->pcolname + i * SQLCOLLEN);
    if (colname_len == 0) {
        // given column name if there isn't
        strcpy(result->pcolname + i * SQLCOLLEN, dft_col_name.c_str());
        colname_len = dft_col_name.size();

        /*!! Confusion !!*/

        std::cout << dft_col_name << '\n';
        // O2 print: lots of corrupted text
        std::cout << dft_col_name.c_str() << '\n';
        // O2 print: "NONE"
        printf("%lu %lu %s\n", colname_len, dft_col_name.size(), dft_col_name.c_str());
        // -g O0 print: 4 4 NONE
        // O2 print: 17179869184 17179869184 NONE

    }
    result->plen[i] = colname_len;
}

一些调试信息复制为：

66                  std::cout << dft_col_name.size() << '\n';
(gdb) 
17179869184
67                  colname_len = dft_col_name.size();
(gdb) p dft_col_name 
$1 = ""
(gdb) p dft_col_name .size()
$2 = 4
(gdb) p dft_col_name.c_str()
$3 = 0x659c98 "NONE"

什么：result

struct ResultInfo{
...
int16_t colnum; // SQL result column number. It's 1 in this problem.
...
int16_t* pcolname; // SQL result columns' name. Each has SQLCOLEN characters.
...
};

#define SQLCOLEN 64

它是从网络反序列化的，在内存中它看起来像：

| struct ResultInfo | <pcolname point to here> colnum * 64 char |

环境：

Linux version 3.10.0-1062.12.1.el7.x86_64 ([email protected])
(gcc version 4.8.5 20150623 (Red Hat 4.8.5-39) (GCC) ) #1 SMP Tue Feb 4 23:02:59 UTC 2020

g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

我真的很困惑！

我尝试使用调试，但没有找到。valgrind --tool=memcheckInvalid write

我试着把、和重新绑在一起，它起作用了。const std::stringconst char[]dft_col_name.size()strlen()

我尝试从类成员中删除作为中的全局变量，或中的局部变量。它没有用。static const std::string a_class::dft_col_nameconst std::string dft_col_name.cppa_class::function()

C++11 编译 stdstring

const std：：string：：size（）比优化级别 2 中应有的要大得多

const std::string::size() is much larger than it should be in optimization level 2

评论

const std：：string：：size（） 比优化级别 2 中应有的要大得多

const std::string::size() is much larger than it should be in optimization level 2

评论

const std：：string：：size（）比优化级别 2 中应有的要大得多