Clever Geek Handbook

C ++ How to print the value of a String attribute of a variable of a different type?

I want to display a C ++ value String

in the console, but this particular one is String

not defined independently - it is an attribute of a variable of a different type ... The line I am currently trying to display its value is:

printf("\n CSARSixSectorItem.cpp line 530. rm_WPSequence[liSARIndex -1]: %s", rm_WPSequence[liSARIndex-1].rm_RefPointDB->m_Name);

Outside of the quotes, I pass the variable: rm_WPSequence[liSARIndex-1].rm_RefPointDB->m_Name

in %s

to printf

.

m_Name

is a variable of type String, defined with:

std::string m_Name;

rm_RefPointDB

is a pointer to CGenericRefPoint

, defined with:

CGenericRefPoint * rm_RefPointDB;

and rm_WPSequence is vector

defined with:

vector< CUserWaypointListElement > rm_WPSequence;

However, although the actual variable I am trying to display is String

when the line is printed to the console, I am not given the contents of the line, but some unreadable characters such as L,%

... The displayed characters change every time the line is printed. I am wondering if this is because String

memeber is another variable? If so, how can I display the String value? I don't really want to know anything about its parent variables, but is there something else I need to do in order to access the String myself?

+3


source to share


2 answers


Use the c_str () function of std to jump to% s:

printf("\n CSARSixSectorItem.cpp line 530. rm_WPSequence[liSARIndex -1]: %s", rm_WPSequence[liSARIndex-1].rm_RefPointDB->m_Name.c_str());


Explanation:% s expects a char * to display. You are passing a std :: string, which is the type of the class.

c_str () returns a pointer to the char * buffer contained in std :: string.

+2


source


Decision

When printing std::string

using format printf

%s

, you should use the method c_str()

std::string

like this:

rm_WPSequence[liSARIndex-1].rm_RefPointDB->m_Name.c_str()

You are using the C-style output method printf

, so the C-style parameter must be used to print the line.

c_str()

is a member function std::string

. It returns a pointer to an array that contains a numm-terminated character sequence representing the current value of the string object. This is the actual array char

that contains the name string character. This is the C-style line,

Pay attention to compiler warnings or errors

For this problem, the latest stable compiler should give you an error. Please note that this can save you a lot of time.



For the following test code, both clang ++ and g ++ will throw an error. They do not allow passing an object non-POD

like a ...

function call printf

.

#include <string>
#include <cstdio>
#include <iostream>

using namespace std;
int main() {
  string a("This is a string");
  printf("0x%x, 0x%s, 0x%x\n", a, a , &a);
  cout << a << endl;
  return 0;

}
Clang ++ exit 
$ clang++ Stringcstring.cpp 
Stringcstring.cpp:26:32: error: cannot pass non-POD object of type 'string' (aka 'basic_string<char>') to
      variadic function; expected type from format string was 'unsigned int' [-Wnon-pod-varargs]
  printf("0x%x, 0x%s, 0x%x\n", a, a , &a);
            ~~                 ^
Stringcstring.cpp:26:35: error: cannot pass non-POD object of type 'string' (aka 'basic_string<char>') to
      variadic function; expected type from format string was 'char *' [-Wnon-pod-varargs]
  printf("0x%x, 0x%s, 0x%x\n", a, a , &a);
                  ~~              ^
Stringcstring.cpp:26:35: note: did you mean to call the c_str() method?
  printf("0x%x, 0x%s, 0x%x\n", a, a , &a);
                                  ^
                                   .c_str()
Stringcstring.cpp:26:39: warning: format specifies type 'unsigned int' but the argument has type
      'string *' (aka 'basic_string<char> *') [-Wformat]
  printf("0x%x, 0x%s, 0x%x\n", a, a , &a);
                        ~~            ^~
1 warning and 2 errors generated.
g ++ output 
$ g++ Stringcstring.cpp 
Stringcstring.cpp: In function ‘int main()’:
Stringcstring.cpp:26:41: error: cannot pass objects of non-trivially-copyable type ‘std::string {aka class std::basic_string<char>}’ through ‘...’
   printf("0x%x, 0x%s, 0x%x\n", a, a , &a);
                                         ^
Stringcstring.cpp:26:41: error: cannot pass objects of non-trivially-copyable type ‘std::string {aka class std::basic_string<char>}’ through ‘...’
Stringcstring.cpp:26:41: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 4 has type ‘std::string* {aka std::basic_string<char>*}’ [-Wformat=]

Why did you see the output of meaningless characters

your object rm_WPSequence[liSARIndex-1].rm_RefPointDB->m_Name

is a class object std::string

. For the format printf

%s

, what it accepts is just an array char

, or it will try to explain your object std::string

in memory as char *

, which is why you saw the output of meaningless characters.

0


source






More articles:


All Articles