为什么我不能按名称访问列

Question

示例代码：

pqxx::connection c("user=postgres");
pqxx::work txn(c);
pqxx::result r = txn.exec("SELECT d.datname as "Name","
       "pg_catalog.pg_get_userbyid(d.datdba) as "Owner","
       "pg_catalog.pg_encoding_to_char(d.encoding) as "Encoding","
       "d.datcollate as "Collate","
       "d.datctype as "Ctype","
       "pg_catalog.array_to_string(d.datacl, E'\n') AS "Access privileges""
       "   FROM pg_catalog.pg_database d"
       "   ORDER BY 1");
try {
  for(int rownum=0; rownum<r.size(); ++rownum ) {
    const pqxx::result::tuple row = r[rownum];
    std::cout << "Column 0 Name: '" << row[0].name() << "': " << row[0].c_str() << std::endl;
    const std::string s = "Name";
    std::cout << (row[0].name() == s) << std::endl;
    std::cout << "dbname: '" << row[s].c_str() << "'" << std::endl; // Exception here
  }
} catch(const pqxx::argument_error &e) {
  std::cout << "Argument Error: " << e.what() << std::endl;
}

我的输出如下

Column 0 Name: 'Name': mydb
1
Argument Error: Unknown column name: 'Name'

第一列的名称是"name"，根据字符串"name"测试行名称会生成一个true语句。但当我用那个字符串访问它时，我会得到一个异常。

Answer 1

好的，完全愚蠢。如果你想让大写字母之类的东西起作用，你必须引用名字。

所以改变

const std::string s = "Name";

至

const std::string s = ""Name"";

修复了问题。显然，C接口降低了您所称的大小写，但没有降低测试所针对的字段的大小写。ARG！我必须遍历libpqxx代码才能弄清楚。