C++ error: no viable conversion from 'mapped_type' to 'int'

Question

I am trying to implement the a map from the C++ STL as follows:

#include <string>
#include <iostream>
#include <map>
using namespace std;
#include "assembler.h"

// This Class makes use of the Map Template from the Standart Template Library
// All addresses are stored as numerical (Dec) integers

SymbolTable::SymbolTable() { // Constructor
    map <string, int> symbolTable;
    int address = 0;
}

void SymbolTable::addEntry(string symbol, int address) {
symbolTable[symbol] = address;
address++;
}

// Returns true if symbolTable already contains symbol
bool SymbolTable::contains(string symbol) {
    if (symbolTable.find(symbol) == symbolTable.end()) { return true; }
    else { return false; }
}

int SymbolTable::getAddress(string symbol) {
    return symbolTable[symbol];
}

I try to compile this with

c++ *.cpp -0 assembler.out

and I get the following error message:

symboltable.cpp:57:9: error: no viable conversion from 'mapped_type' (aka 'std::basic_string<char>') to 'int'
    return symbolTable[symbol];
           ^~~~~~~~~~~~~~~~~~~
1 error generated.

I have searched for this error online and all I get is bug reports relating to the STL and I cannot figure out if those reports are the same problem I am having and if so how to get around it. Am I doing something wrong?

I have tried (probably stupidly) to typecast the offending line as

return (int) symbolTable[symbol];

Thank you for any help.

My header file declares the class as:

class SymbolTable {
public:
    SymbolTable();
    void addEntry(string, int); 
    bool contains(string);
    int getAddress(string);
private:
    map <string, string> symbolTable;
    int address;
};

Please show your class body (i.e. `class SymbolTable { ...`). — Oliver Charlesworth, May 22 '12 at 23:14
@Justin: I assume Oli wants the class definition because you appear to have made (in almost all of your functions) local copies of class members, which won't do what you want it to at all. It appears that the class definition has an incorrect definition of symbolTable. — Mooing Duck, May 22 '12 at 23:15

score 3 · Accepted Answer · answered May 22 '12 at 23:15

3

This:

SymbolTable::SymbolTable() { // Constructor
    map <string, int> symbolTable;
                        ^
                        ^

is a function-local variable, not a member variable. It is not the same as the symbolTable that you're accessing in e.g. getAddress, which presumably is a member variable. You haven't shown the class body, but my guess is that it's defined differently.

answered May 22 '12 at 23:15

Oliver Charlesworth

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Even though I declared it in the .h file? Sorry just started with C++ – jds May 22 '12 at 23:17
Also `int address = 0;` in the constructor. `address++;` in the `add` function also increments the wrong `address` variable. – Mooing Duck May 22 '12 at 23:17
@Justin: Yes. Local variables hide member variables. – Oliver Charlesworth May 22 '12 at 23:17
@Justin: That's not how constructors work, your code creates a new variable local to the function that has the same name as your class member, and then does nothing with it. Read up on ["initializer lists"](http://stackoverflow.com/questions/926752/why-should-i-prefer-to-use-member-initialization-list) – Mooing Duck May 22 '12 at 23:18
@Justin, the term to google about local and class members is "shadowing". – chris May 22 '12 at 23:18

C++ error: no viable conversion from 'mapped_type' to 'int'

1 Answers1