How can I use the compile time constant __LINE__ in a string?

Question

I can use __LINE__ as a method parameter just fine, but I would like an easy way to use it in a function that uses strings.

For instance say I have this:

11    string myTest()
12    {
13     if(!testCondition)
14       return logError("testcondition failed");
15    }

And I want the result of the function to be:

"myTest line 14: testcondition failed"

How can I write logError? Does it have to be some monstrosity of a macro?

Answer 1

There's no reason to do any run-time work for this:

#include <iostream>

// two macros ensures any macro passed will
// be expanded before being stringified
#define STRINGIZE_DETAIL(x) #x
#define STRINGIZE(x) STRINGIZE_DETAIL(x)

// test
void print(const char* pStr)
{
    std::cout << pStr << std::endl;
}

int main(void)
{
    // adjacent strings are concatenated
    print("This is on line #" STRINGIZE(__LINE__) ".");
}

---

Or:

#define STOP_HAMMER_TIME(x) #x
#define STRINGIFICATE(x) STOP_HAMMER_TIME(x)

If you're a cool person like James.

Answer 2

Why do you even need it as a string? What's wrong with an integer? Here are two ways you could write logError() :

#define logError(str) fprintf(stderr, "%s line %d: %s\n", __FILE__, __LINE__, str)

// Or, forward to a more powerful function
#define logError(str) logError2(__FILE__, __LINE__, str)
void logError2(const char *file, int line, const char *str);

If you really need the line as a string, you can use the stringizing operator # , but because of the way macros work, you'll need to wrap it in two macros:

#define STRINGIZE(x) STRINGIZE2(x)
#define STRINGIZE2(x) #x
#define LINE_STRING STRINGIZE(__LINE__)

And now LINE_STRING is a macro that will expand to a string containing the current line number wherever it is expanded. If you only had one level of macros (ie if you had #define STRINGIZE(x) #x ), then you would get the literal string "__LINE__" every time you expanded it, which is not what you want.

Answer 3

His goal is to create a macro (named logError ) that will automatically include the symbols necessary and do the string concatenation within the preprocessor, only using string literals.

So, combining the basically-correct answers answers thus far, let's write the macro:

#define STRINGIZE_DETAIL(x) #x
#define STRINGIZE(x) STRINGIZE_DETAIL(x)
#define logError(msg) (__FILE__ " line " STRINGIZE(__LINE__) ": " msg)

You can then use this macro anywhere to create a generic error message code in string literal format at compile time.

Note: You can also use __FUNCTION__ (or an equivalent, it varies by compiler) instead of __FILE__ , if you prefer, to keep track of the function name instead of the file name.

Answer 4

The usual options for formatting a number into a string apply: Boost lexical_cast, ostringstream, sprintf or snprintf, etc.

Here is one of my favorite links on the topic: http://www.gotw.ca/publications/mill19.htm

Answer 5

Yes, it's ugly. You need a combination of macros. Converting an integer to a string is a two-step process - here's Boost's implementation:

#define BOOST_STRINGIZE(X) BOOST_DO_STRINGIZE(X)
#define BOOST_DO_STRINGIZE(X) #X

Now you can generate a string:

logError(__FILE__ BOOST_STRINGIZE(__LINE__) "testcondition failed");   

Answer 6

std::string logError(const char* file, int line, const char* msg)
{
   std::ostringstream os;
   os << file << ' ' << line << ':' << msg;
   return os.str();
}

Usage:

return logError(__FILE__, __LINE__, "my error message");

You could then make a macro for this if you were so inclined:

#define LOG_ERROR(x) logError(__FILE__, __LINE__, (x))

And then the usage would be:

return LOG_ERROR("my error message");

Answer 7

sprintf(newStringBuffer, "myTest line %d: testcondition failed\n", __LINE__);

should do it c style. I know that there are ways and ways of doing this with the C++ string libraries.

You could also use strcat() or strncat or any other number of C libs to do this.

cout <<"String" + __LINE__ + " another string" 

will work as well.

Answer 8

Try this?

string myTest(const int lineno)
{
  if(!testCondition)
    return logError ("testcondition failed", lineno);
}

void logError (string msg, const int lineno)
{
  clog << "line " << lineno << ": " << msg << endl;
}