C/C++ line number

Question

You should use the preprocessor macro __LINE__ and __FILE__. They are predefined macros and part of the C/C++ standard. During preprocessing, they are replaced respectively by a constant string holding an integer representing the current line number and by the current file name.

Others preprocessor variables :

• __func__ : function name (this is part of C99, not all C++ compilers support it)
• __DATE__ : a string of form "Mmm dd yyyy"
• __TIME__ : a string of form "hh:mm:ss"

---

Your code will be :

if(!Logical)
  printf("Not logical value at line number %d in file %s\n", __LINE__, __FILE__);

As part of the C++ standard there exists some pre-defined macros that you can use. Section 16.8 of the C++ standard defines amongst other things, the __LINE__ macro.

__LINE__: The line number of the current source line (a decimal constant).
__FILE__: The presumed name of the source file (a character string literal).
__DATE__: The date of translation of the source file (a character string literal...)
__TIME__: The time of translation of the source file (a character string literal...)
__STDC__: Whether__STDC__ is predefined
__cplusplus: The name __cplusplus is defined to the value 199711L when compiling a C ++ translation unit

So your code would be:

if(!Logical)
  printf("Not logical value at line number %d \n",__LINE__);

You could use a macro with the same behavior as printf(), except that it also includes debug information such as function name, class, and line number:

#include <cstdio>  //needed for printf
#define print(a, args...) printf("%s(%s:%d) " a,  __func__,__FILE__, __LINE__, ##args)
#define println(a, args...) print(a "\n", ##args)

These macros should behave identically to printf(), while including java stacktrace-like information. Here's an example main:

void exampleMethod() {
    println("printf() syntax: string = %s, int = %d", "foobar", 42);
}

int main(int argc, char** argv) {
    print("Before exampleMethod()...\n");
    exampleMethod();
    println("Success!");
}

Which results in the following output:

main(main.cpp:11) Before exampleMethod()...
exampleMethod(main.cpp:7) printf() syntax: string = foobar, int = 42
main(main.cpp:13) Success!

Use __LINE__ (that's double-underscore LINE double-underscore), the preprocessor will replace it with the line number on which it is encountered.

C++20 offers a new way to achieve this by using std::source_location. This is currently accessible in gcc an clang as std::experimental::source_location with #include <experimental/source_location>.

The problem with macros like __LINE__ is that if you want to create for example a logging function that outputs the current line number along with a message, you always have to pass __LINE__ as a function argument, because it is expanded at the call site. Something like this:

void log(const std::string msg) {
    std::cout << __LINE__ << " " << msg << std::endl;
}

Will always output the line of the function declaration and not the line where log was actually called from. On the other hand, with std::source_location you can write something like this:

#include <experimental/source_location>
using std::experimental::source_location;

void log(const std::string msg, const source_location loc = source_location::current())
{
    std::cout << loc.line() << " " << msg << std::endl;
}

Here, loc is initialized with the line number pointing to the location where log was called. You can try it online here.

Checkout __FILE__ and __LINE__ macros