Question

What is the meaning of this code written in c++?

PGBulkInserter pgBulkInserter(postgreSQL, HOST_TRAFFIC_SCHEMA_NAME, date,
                            flushSize, "%ud, %ud, %ud, %ud, %ud, " \
                            "%ul, %ul, %Vul, %Vud, %Vud, %Vud, %Vul, " \
                            "%Vud, %Vud, %Vud, %Vud, %Vud, %ud, %ud, " \
                            "%ud, %ud, %ud");

I understand that it is creating an instance of PGBulkinserter, but what is the meaning of %ul, %ud %vud, etc.? Would you explain the deeper meaning (i.e., how many parameters PGbulkinsterter has, etc.)?

I think they are like types %d, %f, etc. Does anyone knows if %ud means an unsigned version of %d (I’m just guessing).

Edit: I’m pretty sure that %ud is a 32 bit unsigned decimal, which is used for time in another piece of code. Also, "%ul, %ul, %Vul, %Vud, %Vud, %Vud, %Vul, " is related to the type for all entries in the table. Now, the question is what is \?

Answer 1

If this is supposed to be an initialization (a constructor call), then it passes 5 arguments to the constructor. The last argument is a string literal. It has no C++-specific meaning. It’s meaning is determined by the function. Read the function documentation to figure out what it means. It looks similar to C-style format string (fscanf/fprintf), but the actual specifiers (like %Vud) are not standard.

The last argument is really a single string literal split across multiple lines. C++ language concatenates consecutive string literals (i.e literals separated by whitespace) into a single literal. For example this sequence of seemingly separate literals

"Hel" "lo" " worl" 
"d"

is actually interpreted by the compiler as a single literal

"Hello world"

The \ character at the end of the line causes the preprocessor to combine several lines of source file into a single line. For that the \ character must be the last character on the line (as in your example). For example, this

in\
t\
  ma\
in(\
)

is equivalent to

int main()

In other words, your code sample is really equivalent to

PGBulkInserter pgBulkInserter(postgreSQL, HOST_TRAFFIC_SCHEMA_NAME, date, flushSize, 
  "%ud, %ud, %ud, %ud, %ud, %ul, %ul, %Vul, %Vud, %Vud, %Vud, %Vul, %Vud, %Vud, %Vud, %Vud, %Vud, %ud, %ud, %ud, %ud, %ud");

The author of the code apparently believed that in order to make the compiler to concatenate literals, they have to be forced into a single line first. For that they used the \ character. However, it was completely unnecessary, since the literals would have been concatenated anyway even without the \.

In your specific code sample you can completely remove the \ characters at the end of each line, and it will have absolutely no effect on the program. Sometimes people use those \ anyway just to draw attention to he fact that the literal continues on the next line.