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C++ lambda function error, while implementing dfs (closed)
By angry_pikachu, history, 2 years ago, In English

I am getting error

prog.cpp: In lambda function:

prog.cpp:118:28: error: use of ‘dfs’ before deduction of ‘auto’

118 | for(ll it: nde[i]) dfs(it,fren[i]);

In this code , ll means long long, nde[i] is a vector of int 

auto dfs = [&](int i,int x)->void{
        fren[i]+=x;
        for(ll it: nde[i]) dfs(it,fren[i]);
    };
dfs(0,0);

Writing a blog in codeforces for first time

C++ 17 online compiler

Worked using 

    function<void(int, int)> dfs = [&](int i, int x) -> void {
        fren[i] += x;
        for (ll it : nde[i])
            dfs(it, fren[i]);
};

Also by changing vector<int> nde[n] to vector<vector<int>> nde(n) and by using the function 

    auto dfs = [&](int i,int x, auto &&dfs)->void{
        fren[i]+=x;
        for(ll it: nde[i]) dfs(it,fren[i], dfs);
    };
    dfs(0,0,dfs);
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Comments Comments (8)
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2 years ago, # |
  Vote: I like it +4 Vote: I do not like it

Correct it to this

auto dfs = [&](int i,int x, auto &&dfs)->void{
        fren[i]+=x;
        for(ll it: nde[i]) dfs(it,fren[i], dfs);
    };
dfs(0,0,dfs);
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    2 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    Getting this error

    prog.cpp: In function ‘void solve()’:

    prog.cpp:116:16: error: use of deleted function ‘solve()::<lambda(int, int, auto:22&&)>::~()’

    116 | auto dfs = [&](int i,int x, auto &&dfs)->void{

    117 | fren[i]+=x;

    118 | for(ll it: nde[i]) dfs(it,fren[i], dfs);

    119 | };

    prog.cpp:116:18: note: ‘solve()::<lambda(int, int, auto:22&&)>::~()’ is implicitly deleted because the default definition would be ill-formed:

    116 | auto dfs = [&](int i,int x, auto &&dfs)->void{

    |

    Have to edit the comment due to multi line issue :(

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      2 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      can you share the whole code?

      I think in the above code ll (long long) is causing the issue as it is getting typecasted to int as a reference argument.

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        2 years ago, # ^ |
          Vote: I like it -8 Vote: I do not like it

        Yes 

        #include <bits/stdc++.h>
using namespace std;
/* DEBUGGING */
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
    cerr << '{';
    __print(x.first);
    cerr << ',';
    __print(x.second);
    cerr << '}';
}
template <typename T>
void __print(const T &x) {
    int f = 0;
    cerr << '{';
    for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
    cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
    __print(t);
    if (sizeof...(v)) cerr << ", ";
    _print(v...);
}
#ifndef ONLINE_JUDGE
#define deb(x...)                 \
    cerr << "[" << #x << "] = ["; \
    _print(x)
#else
#define deb(x...)
#endif
/* MACROS */
typedef long long int ll;
 
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;
 
typedef map<int, int> mii;
typedef tuple<int, int, int> tup;
 
#define ff first
#define ss second
#define pb push_back
#define IOS                           \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL);
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        // http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
// eg - unordered_map<long long, int, custom_hash> safe_map; // for safe hash
#define lb lower_bound
#define ub upper_bound
#define all(x) (x).begin(), (x).end()
#define sz(x) (x).size()
#define ps(x, y) fixed << setprecision(y) << x
#define setbit(x) __builtin_popcountll(x)
#define rep(i, a, b) for (int i = a; i < b; ++i)
#define repr(i, a, b) for (int i = a; i >= b; --i)
 
/* CONSTANTS */
#define PI 3.141592653589793
const ll MOD = 1e9 + 7;
const ll INF = 1000000000;
const ll MAX_N = 2e5 + 2;
ll add(ll x, ll y) {ll res=x+y; return(res>=MOD?res-MOD:res);}
ll mul(ll x, ll y) {ll res=x*y; return(res>=MOD?res%MOD:res);}
ll sub(ll x, ll y) {ll res=x-y; return(res<0?res+MOD:res);}
ll power(ll a,ll b,ll m=MOD){ ll ans=1; a=a%m;  while(b>0) {  if(b&1)  ans=(1ll*a*ans)%m; b>>=1;a=(1ll*a*a)%m;}return ans;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
ll lcm( ll x, ll y) { return (x*y)/gcd(x,y);}
bool isprime(ll n){if(n < 2) return 0; ll i = 2; while(i*i <= n){if(n%i == 0) return 0; i++;} return 1;}
 
void solve() {
    ll n; cin >> n;
    vll en(n), fren;  rep(i,0,n)cin>>en[i];
    fren = en;
    vll nde[n];
    rep(i,0,n-1){
        ll x, y; cin>>x>>y;
        x--,y--;
        nde[x].pb(y);
        if(en[x])fren[y]++;
        if(en[y])fren[x]++;
    }
    auto dfs = [&](int i,int x)->void{
        fren[i]+=x;
        for(ll it: nde[i]) dfs(it,fren[i]);
    };
    dfs(0,0);
    ll q; cin >> q;
    while(q--){
        ll x, t, k; cin>>x>>t>>k;
    }
    rep(i,0,n) cout<<fren[i]<<" "; cout<<'\n';
}
 
int main() {
    IOS;
    ll t = 1;
    // cin >> t;
    for(int i = 1; i <= t; i++){
        //cout << "Case #<< i << " ";
        solve();
    }
}
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          2 years ago, # ^ |
            Vote: I like it +11 Vote: I do not like it

          Try removing the VLA. A lot of people I know face this same error because they use VLAs. More precisely, do something about the vll nde[n]; and convert it to a vector of vectors. VLAs are gcc extensions and are discouraged, and are not legal C++ at all.

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            2 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            vector<vll> nde(n);

            I have used this, still same error :(

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              2 years ago, # ^ |
                Vote: I like it 0 Vote: I do not like it

              You also have to do the auto&& dfs change that was mentioned in the top level comment.

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2 years ago, # |
← Rev. 2   Vote: I like it +12 Vote: I do not like it

Try this 

function<void(int, int)> dfs = [&](int i, int x) -> void {
        fren[i] += x;
        for (ll it : nde[i])
            dfs(it, fren[i]);
};
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