Re: Should one use std::string*, std::map* to avoid copying ?

"Daniel T." <>
Tue, 08 Jan 2008 18:28:54 -0500
digz <> wrote:

This is a very simplified version of something I am trying to
The State object holds the strings and maps , and I pass a reference
to State to the process Function which manipulates it based on the tag
passed in.

I cannot pass the strings/maps in question as references coz ..
a) thats redundant, state has everything, it increases number of
parameters unnecessarily
b) I have to do the case/switch outside the function to decide what to
pass in.( and this is a simple example )

Is it a good idea to use things like string* and std::map..* as i have
tried below

I'd like to see how else this State object is used before commenting too
much. However the first thing I would do is break the function up a bit.

void processA( string& prodId, mapType& typeSymMap )
   // why so short? See notes below...
   if ( ! typeSyMap.empty() )
      prodId = typeSymMap.begin()->second;

void process( State& state, int tag )
   switch ( tag ) {
   case 0:
      processA( state.pId_, state.pTypeNSymbol_ );
   case 1:
      processA( state.uId_, state.uTypeNSymbol_ );
      assert( false && "Bad tag value" );

Still doesn't make much sense, but I don't know the problem domain.
Comments below:


using namespace std;
typedef std::map<int, string> mapType;

struct State {
  string pId_;
  mapType pTypeNSymbol_;
  string uId_;
  mapType uTypeNSymbol_;

Can you come up with some better names? Can you come up with some member
functions? The 'process' function below would be an ideal

void process( State& state, int Tag){
  typedef mapType::iterator mItr ;
  string* prodId;
  mapType* typeSymMap;

Initialize your variables at the point of definition.

  switch( Tag ) {
    case 0:
      prodId = &state.pId_;
      typeSymMap = &state.pTypeNSymbol_;
    case 1:
      prodId = &state.uId_;
      typeSymMap = &state.uTypeNSymbol_;

What if Tag is neither 0 or 1? If it can only be 0 or 1, then why not a

  if (prodId->empty()) {
    mItr it;

Again, initialize your variables at the point of definition. However,
this variable "it" is completely pointless so remove it.

    mItr end = typeSymMap->end();

Remove "end" as well. Extraneous variables cause headaches.

      if ( (it = typeSymMap->find(6)) != end){
        *prodId = (*typeSymMap)[6];

  if (!typeSymMap->empty()) {
    mItr it = typeSymMap->begin();

Another extraneous variable. At least this one is initialized...

    *prodId = it->second;

For the two 'if' statements above... If typeSymMap.empty() == false,
then the final result will be *prodId == typeSymMap->begin()->second. If
typeSymMap.empty() == true then there won't be a key of '6' in it so the
first 'if' statement (where it tries to find a key of '6') is utterly

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