C++ macro adding to std::map

Question

I'm very new to C++. I just started to pick it up this last week or so.

I am trying to write a macro to be able to access and add items to a std::map. However, I'm not finding this to work. What am I doing wrong or is there a better way to implement this?

#include <iostream>
#include <string>
#include <map>

typedef struct {
  std::string name;
  int value;
} Token;

std::map<std::string, Token> ALL_TOKENS;

#define T(macro, name, value) \
  Token macro {name, value}; \
  ADD(name, macro); \
  T(TEST, "Test", 1); \

#define ADD(str, tok) \
  ALL_TOKENS.insert(std::pair<std::string, Token>(str, tok)); \

#define GET(str) \
  ALL_TOKENS[str]; \

int main(int argc, const char * argv[]) {
  Token tok = GET("Test");
  printf("%s", tok.name.c_str());
  return 0;
}

Is there a better way? I can hardly think of a worse way... — Kerrek SB
– Kerrek SB, Commented Nov 16, 2014 at 2:21
@KerrekSB Ok...? I'm new to c++. I just picked it up last week. — Chase W.
– Chase W., Commented Nov 16, 2014 at 2:23
Looks like you created more work with the macros than you are benefiting from. — Thomas Matthews
– Thomas Matthews, Commented Nov 16, 2014 at 2:51

Jerry Coffin · Accepted Answer · 2014-11-16 02:56:02Z

2

In my opinion, your macros make the code more difficult to understand. It also looks to me like you don't entirely understand std::map yet.

When I use map, I usually use operator[] for insertion, and possibly for retrieving as well. It's usually easiest if you think of it just as an array that can use essentially any (sortable) type as a subscript.

For example, to count how often each word occurs in a file, you could do something like this:

std::string word;
std::map<std::string, size_t> counts;

while (infile >> word)
    ++counts[word];

Then you could print out all the words and how often each was found (in alphabetical order) with code something like this:

for (auto const &w : counts)
    std::cout << w.first << "\t" << w.second << "\n";

At least to me, this seems simpler and more understandable than any code I could write using your macros.

answered Nov 16, 2014 at 2:56

Jerry Coffin

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1 Comment

Chase W. Over a year ago

Thanks you're response helped the most.

R Sahu · Accepted Answer · 2014-11-16 02:43:46Z

0

Disregarding the poor choice of using macros to access functionality...

You are not getting any output since you have not added anything to the map.

Try

int main(int argc, const char * argv[]) {
  Token t{"Test", 10};
  ADD("Test", t);
  Token tok = GET("Test");
  printf("%s\n", tok.name.c_str());
  return 0;
}

You'll get some output.

answered Nov 16, 2014 at 2:43

R Sahu

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