C++ Lambda -- String params not working in following function calls

Question

I'm having an issue where when I try to access a string that's the main param in a lambda, the compiler doesn't recognize it as so when I try to call functions using that string.

Here is my code:

void removePunctuation(std::vector<std::string> &inTokens,
                   std::vector<std::string> &outTokens) {
std::for_each(inTokens.begin(), inTokens.end(), [outTokens](std::string s) {
    std::string newS = s;
    // newS.erase(std::remove_if(newS.begin(), newS.end(), ispunct));
    outTokens.push_back(newS);});
}

And the following error is produced:

a2.cpp:114:19: error: no matching member function for call to 'push_back'
    outTokens.push_back(newS);});

I am also getting this sort of error in other functions, when I try to call a function that uses the lambda's string param in its call.

Any help is much appreciated!

Did you mean `std::for_each(inTokens.begin(), inTokens.end(), [&outTokens](const std::string& s) {std::string newS = s; outTokens.push_back(newS);});` — Simon Kraemer, Oct 13 '16 at 16:16

score 5 · Answer 1 · edited May 23 '17 at 12:00

5

By default, lambda arguments are passed as read-only, try

[&outTokens](std::string s)

(It is perhaps what you want anyway, if the outTokens parameter is expected to be modified.)

edited May 23 '17 at 12:00

Community

1
1

answered Oct 13 '16 at 16:17

AlexD

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Slava · Answer 2 · 2016-10-13T16:30:58.573

2

Aside that you should capture outTokens by reference, std::transform suits this better:

outTokens.resize( inTokens.size() );
std::transform(inTokens.begin(), inTokens.end(), outTokens.begin(),
    [](std::string s) {
        s.erase(std::remove_if(s.begin(), s.end(), ispunct));
        return s;
    } );

If you pass std::string by value you do not need to create yet another copy to modify it and first parameter to removePunctuation should be const reference.

edited Oct 13 '16 at 16:30

answered Oct 13 '16 at 16:23

Slava

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However, now it seems to be erasing every single token that I end up printing. My call is as follows: while (getline(inFile, line)) { replaceHyphensWithSpaces(line); std::vector initialWords, words; splitLine(line, words); removePunctuation(initialWords, words); – Oct 13 '16 at 17:09
I put there exact logic as you showed in your code, if you need it work different way create another question – Slava Oct 13 '16 at 21:34

score 1 · Answer 3 · answered Oct 13 '16 at 16:18

You should capture outTokens by reference:

void removePunctuation(std::vector<std::string> &inTokens,
                   std::vector<std::string> &outTokens) {
    std::for_each(inTokens.begin(), inTokens.end(), [&outTokens](std::string s)
       {
           std::string newS = s;
           outTokens.push_back(newS);
       });
}

score 1 · Answer 4 · edited Jun 20 '20 at 09:12

Variables captured by copy can't be modified, unless you make lambda mutable.

mutable - allows body to modify the parameters captured by copy, and to call their non-const member functions

Unless the keyword mutable was used in the lambda-expression, the function-call operator is const-qualified and the objects that were captured by copy are non-modifiable from inside this operator().

e.g.

[outTokens](std::string s) mutable { ... }

But this means you'll modify on copy of outTokens, which might not be your intent. (Modify on a copy and then return doesn't make much sense.) You might want change it to capture by reference:

[&outTokens](std::string s) { ... }

C++ Lambda -- String params not working in following function calls

4 Answers4