C/C++ line number
In the sake of debugging purposes, can I get the line number in C/C++ compilers? (standard way or specific ways for certain compilers)
e.g
if(!Logical)
printf("Not logical value at line number %d \n",LineNumber);
// How to get LineNumber without writing it by my hand?(dynamic compilation)
Solution 1:
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__);
Solution 2:
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__);
Solution 3:
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!
Solution 4:
Use __LINE__
(that's double-underscore LINE double-underscore), the preprocessor will replace it with the line number on which it is encountered.
Solution 5:
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.